Long story short:
From their home planet, the aliens can only determine if a star system contains a planet with liquid water and carbon occurence, but they can't see if the planet is really suitable for colonization.
When the aliens hide from humans, they can live on earth for a long time, but since humans are a pretty aggressive species, the aliens wouldn't survive a colonization. So they move on to the next star.
Like most living beings, the aliens have a survival instinct. They want to have a "backup" of them, so they start several spaceships from their home planet to the nearest star systems. Their technology is only a few hundred years ahead of human technology, so they share some of the problems that humans have with interstellar travel. FTL travelling isn't possible, cryogenic tanks are science fiction or can at least not be used for hybernating an adult alien for several years and transporting living aliens over several generations isn't possible, too (they can't produce enough food for all aliens and the risk of one alien going crazy and destroying the ship during flight is too high).
Fortunately, the aliens got an idea on how to avoid those problems. They simply don't transport living beings on their spaceships. The spaceships are automated transporters that contain storage devices in which the DNA of the alien flora and fauna is stored, alongside many books, videos, websites etc. from the alien planet. Furthermore the aliens edited their DNA to become more intelligent, peaceful and able to settle on planets that are not exactly like their home planet. For example they can withstand wide varieties of gravitational forces, temperatures, poisons etc. They not even need a mother to breed. They edited their DNA so they can reproduce like a cross of a jellyfish and a mushroom. They only need to plant a pollen in a flat riverbed, where it begins to grow to some kind of spawn that extracts nutrients from the earth and can breed the aliens. After four months they are ready to be born.
So this is what happened: The aliens started 16 spaceships from their home planet. One of them with the earth as destination. The spaceship is a small automated transporter controlled by a simple artificial intelligence. It only measures around twice the size of a Boeing 747 airplane. When it arrived in earth orbit, it launched three landing vehicles that were distributed evenly over the earths surface. After that, the spaceship proceeded to the Lagrange point between mercury and sun to fill up its antimatter tanks (what takes a few decades).
The landing vehicles successfully landed in the Gila National Forest in New Mexico, the Black Forest in Germany and the Bureya Nature Reserve in Russia. The vehicles contained everything that is needed to raise 8 aliens per ship. For example some robots that are able to plant the alien seeds in nearby riverbeds, carry the alien babies to the landing vehicle and prepare food, some toys, entertainment and learning devices, VR-nanny projectors and so on.
The aliens grew up in the landing vehicles and the surrounding forests. They learned very fast and with the age of three years they started to research which species are living on the planet. They soon found out that there are lots of humans around and that they are using radio broadcast. The aliens were able to receive the broadcast signals and soon understood the language of the humans. They learned about society, and that in the human calendar they seem to be in the year 1932. They also learned that there just recently has been a world war, so humans seem to be aggressive and they have weapons that could also kill the aliens. The aliens decided not to stay on earth and that they will use humans for their "wetware botnet".
The aliens started using their landing vehicles to capture humans. Since they didn't know much about human biology, the anesthetic didn't work as expected and some humans were still conscious when the brain interface was installed. The interface chips were installed on the backside of the tailbone, because the nerves can be accessed easily there. That's why some of the conscious humans reported of having had a rectal examination by the aliens.
The botnet is not only usable for finding the next star to head to. The observations needed for that could easily be made by computer controlled telescopes. The main reason for the human botnet is their problem solving capability. During the flight from the home planet to earth, the spaceship got hit by many micrometeorites. Some of the repairs can be done automatically but some need the ability to find different solutions. The connected humans can easily do the remote controlled repairs while sleeping, but there was a problem. Since the humans were connected to the alien-Wikipedia, they started to dream of alien technology. Most humans weren't able to remember their dreams, but a few started to research their dreams. In the USA, scientists started to build jet engines. In Germany, Konrad Zuse built a first programmable binary computer.
Then things started to get out of hand. The germans started another world war and soon found the alien landing vehicle that landed in the Black Forest. They killed the aliens, took the ship to a military base and tried to figure out how it works. They started the "Haunebu" project which stands for "Halbautomatische Untersuchung natürlicher Eigenschaften bemannter Untertassen" (half-automatic research of natural properties of manned saucers). They also tried to gather informations about alien tech from the VR-nanny projectors, which they called "Vril" ("virtuelle rechnerinduzierte Lehrerinnen", virtual computer-induced teachers). There are rumors, that they even bypassed the controls and flew those saucers by hand, what didn't work well (same as if you tried to fly a Joint Strike Fighter by manually tilting the control surfaces).
The aliens began to become nervous. They were 24 and now a third of them were killed. The spaceship wasn't fully fueled up yet and only the repairs that could be done remote controlled were made. They still had to build some spare parts on earth and install them in the spaceship. The aliens disconnected the human interfaces, but since one of the landing devices was found and partly disassembled, it wasn't remote controlable anymore. So the interface to the alien database still existed near the german landing device. It didn't take long for the german scientist Otto Hahn to dream about nuclear fission. He started to research that topic and gave lectures in the princeton university what lead to the Manhattan Project and later to the drop of nuclear bombs over Hiroshima and Nagasaki.
The aliens couldn't hide anymore. All the years they lived in remote forests but now they had to stop the humans and try their best to hinder humans from nuking the whole planet. In russia, the alien Bō Prad (known as "Skinny Bob") revealed himself to the KGB and offered Boris Jelzin informations about space travel in exchange for the promise to not launch a nuclear rocket.
The next years went better. The war was over, the disassembled landing vehicle was repaired to a state that it could be remote controlled. Since it wasn't save to use for a flight to the main ship, the aliens intentionally crashed it at a remote location near the south pole.
Sadly, one of the New Mexico aliens suffered a stroke while flying over Roswell and crashed. The aliens only have half of the human lifespan, so a 28 year old alien has to expect such illnesses. Since only one of landing vehicles was left and the aliens didn't trust humans enough to commission them to build more landing vehicles (that would mean, that they had to grant humans access to the knowledge of how to build and control an anti-gravity device. The humans would most likely use it for another war.), they repaired the main ship, attached the remaining landing vehicle to it and launched it to the next promising star system.
The earth was very promising but the humans are too dangerous. Since the aliens are a peaceful race, they wouldn't kill all humans. They already have a bad conscience because they frightened some humans with their failed anesthetic. They would have liked to live in a peaceful coexistence but that didn't work out.
The aliens that remained on the earth, helped humans to build better computers and advised them to build a global network of university computers in the future, so humans will get easier access to knowledge. Some of them wrote letters to science fiction authors, to give them some inspiration for their books. The last one of them died 47 years ago in November of 1972. Rest in peace.