system
Given the following post, respond as if you're commenting on Reddit:
human
[star trek]why are most of the big fish in the galaxy (aside from the Borg) only a few thousand years old, warp capability wise? what happened to the races that were prominent in the milky way millions of years ago?
gpt
It is hinted at times that races or ensembles of races that become *too* advanced will become essentially "invisible" for less advanced races. 

For instance the Q became incorporeal, all-powerful extradimensional entities, the "prophets" inside DS9 wormhole live outside time itself, the Fluidic Space is possibly is a pocket dimension created by Species 8742 itself, and so on.  Even the Federation from the 29th century is so involved in time travel shenanigans that they already look kind of "detached" from normal life in the universe.

So, what the Federation normally sees, and what has any consequence or represents any concern for them, are other races that are about in the same window of technological and evolutive development. It's like how ants will perceive just other species of ants and other insects crawling over dirt, almost all the time completely unaware of - say - a gigantic, incredibly more complex and intelligent human observing them from above. The only times that they "perceive" the human (and in a rather incomplete way) are when he accidentally stomps their anthill or starts to burn ants with a magnifying glass for shits and giggles.