Of all places, jRPG give me the most convincing answer, namely Star Ocean series.

The recurring plot element in each Star Ocean game is the UP3. UP3 stands for Underdeveloped Planet Preservation Pact. All advanced space faring races belonging to the federation must avoid direct contact with less developed races unless absolutely necessary in order to preserve the latter's authentic growth.

Makes sense when you think about it. They don't want to just pick up random races and bring them up to their level because all this will accomplish is turn the picked up race into dependants while likely killing off what made them unique in the first place.

Why grow and develop as an individual when you can just leech from your betters?

If said race drives itself to extinction or technological stagnation then it means that they were not mature enough to sit next to other big boys at the galactic table.

So yes, I do think the aliens are just hiding themselves.