Just because the aliens are open with their technology doesn't mean that humans understand it. In the Ringworld universe, Puppeteers build the most powerful ship hulls, but nobody else can. They sell them to other races, but are the only ones who know how to build them. If we gave a microchip to someone from 200 years ago, they would not be able to build another one in 15 years, as another answer implies.
Even though humans may buy the tech from aliens doesn't mean they can afford as much as they want. After all, to buy something, you have to trade something that the other party wants. What kinds of things can humans do that is even useful to such an advanced race? Explaining what humans can offer in trade is the bigger problem, IMO. But whatever it is, it may simply be the case that humans can only afford to buy 1000 starships from the aliens, but they really want one MILLION starships (raise pinky to mouth). If it would take less time to catch up to alien tech levels than it would to earn enough money to buy their natural demand, then it is logical that humans would invest all their resources trying to catch up their own tech than just buying the alien tech.
If your aliens are like the Puppeteers, it could be that they crave safety and what they will pay money for is outsourced risk. So humans may be contracted to carry out risky/dangerous missions, like investigating a star that's about to go supernova, or collecting samples from a novel/corrosive/energetic gas cloud/proto-planet, or just collecting live dangerous creature samples from some planet. Thus, what makes humans useful is that they are disposable, as far as the aliens are concerned. And the aliens are happy to trade their incomprehensible magical artifacts for these risky adventures that humans would be doing anyway, just because we're like that.
Today we take for granted that knowledge is easy to acquire. John Horgan even wrote a book called The End of Science, arguing that we've discovered all the big principles (which must be false if your alien tech exists). What Horgan misses is that scientific principles are just the tiniest first step to building useful products. Technology is actually all about engineering; i.e., the ugly details in putting together something that works. And if you think that's easy, consider that humans have already built numerous technologies that humans themselves don't understand! That is, there is no human that can tell you every aspect of the Google search engine works, or could reproduce it from scratch. The same is true of FB, an iPhone, and even StackExchange itself! Human brains are simply too small to hold all of the necessary information. So while, in principle, a team of engineers could eventually rebuild all of those products with equivalent functionality, the details of the implementation and actual behaviors would be subtly to very obviously different.
But that's just the primitive tech we have today. The real tech is nanotechnology. Humans decided that they would be able to understand biology if they only sequenced the genome, so we completed the Human Genome Project. Then we took a look at the results and said: "Huh. There aren't nearly enough genes here to describe a human body. What gives?" Well, there's lots of problems with the genetic theory, including the fact that our microbiome contributes functionality by creating biomolecules which are used by the rest of the body! So technically, we have outsourced some of our genes to bacteria. Then there's the fact that we call most of the nuclear material "junk DNA" (or introns). But there's a problem with the "junk DNA" hypothesis. You see, DNA is tightly coiled and then squished up into a ball so it fits inside the nucleus (most of the time). But the cell also has to read it, so it also uncoils to do its job. And it turns out that the expression of genes depends on where they are in that 3D ball! Which means that the "junk DNA" may have a functional purpose in shaping the 3D form DNA strands. When it comes to the nanotechnology of life, what we don't know far exceeds what we do.
Take graphene. We've been using elemental carbon since we mastered fire. We used it when we made charcoal, graphite for pencils, diamond drill bits, etc. We even figured out how many protons and neutrons it has, and how all life on earth depends on it. And yet, every day we learn something new about carbon, like how you can form it into tubes, or sheets 1 atom thick. How you can make it into super-capacitors, or even superconductors by twisting two sheets of it by only a few degrees. Knowing about quantum electrodynamics does not directly give this knowledge, nor does quantum chemistry. You have to discover it by playing around with lots of materials, and hoping you stumble upon some happy accidents.
In fact, there is a ridiculous amount of room for an advanced species to create technology based on our currently known laws of physics that would still take us hundreds of years to understand and reproduce ourselves. And much of that comes down to tools. Humans had access to the raw materials to create guns at least 2000 years ago. But whenever someone posts a question here about time travelers and guns, it always comes up that the ancients lacked the machining capabilities to create modern firearms. And so we may even know the principles by which alien tech operates, but lack the ability to reproduce it until we go through the long, hard slog of building up our tooling capabilities to the kinds of things the aliens do.
It would be like showing a schematic of an iCore i9 processor to an electrical engineer from the 60s. Once you explained how the gates work, how lithography works, and all the other things you need to build a modern microprocessor, that poor engineer will just throw up his arms and say: "But it will take us decades to build all that!" Yes, it will. But if you just give that one chip, even embedded inside a laptop, they will only have one laptop. If they want millions of laptops, they need to build up their indigenous tool stack.