Dragon Ball Stuper and it’s lazy writing.
Here’s a list of alien races who’s extinction has not been confirmed, aren't’ native to Earth, or the other world or otherwise part of the god hierarchy.
Frieza Soldier race
Race of Hera
Slug Soldier race
In Dragon Ball Z they gave you this feeling that the universe was teaming with life. I mean who was Freeza selling the planets to?
Even here in the panel from Jaco you get this impression of many worlds working together. I refuse to accept that statement as cannon, the writers just wanted and excuse to focus on Earths’ fighters. Really they could have just said that the strongest worlds are to scattered and that attempting to recruit would take to much to much time.