Make the aliens' thinking biased.
So there are lots of alien life in the universe. They must be sharing something, be it cultural, biological, or even religious, that humans do not have. Imagine if all of your alien species, from all their differents systems, always had two brains. From their point of view, anything with one brain (or less) isn't even considered as sentient. They may aknowledge our very existence, but tagged us as "Primitive - do not bother worry about it".
Then we develop interstellar travel and all of their minds are blowing since for them, it would be like ants having learnt to build spaceships.
Another way to put it would be how they consider a planet suitable for life: "Wow, a planet with a gravity of 9,807 m/s²? Nothing can live there! No need to lose a probe for that!". Atmosphere, temperature, gravity... anything that rings your bell for your story.
And what if humans had contact in the past?
The alien scientist who came here before may have filled reports, maybe some of them have considered us sentients enough to be a part of their... intergalactic Federation, or anything equivalent.
But these reports were dismissed. Either forgotten in the mass of administrative documents, or set aside by some neglectful alien scientist, or worse, because "Humans are not relevant".
"One brain, really? Psssh!"
Humans are f****** terrifying!
Your question reminds me of a short story I read months ago, about an abducted human who is lost in the administrative process of an intergalactic civilisation - unfortunately I can't remember the name of it. At one point, the station is attacked by an arachnid-like creature that can dismember any alien there. But the human isn't even bothered and shred the arachnid to pieces like a doll. And the others aliens are now scared that humans as a species are introduced into their universe. The story has more than that, but for your story it could be enough:
- Humans produce a combat drug when in stress,
- Humans eat organs of other species to gain strenght,
- It is said among the wise ones that if a human bites you, you can turn into a human yourself when the moon is full,
So maybe the aliens are aware of humans. They just don't want them to run everywhere like kids with scissors.