Presumably, some of them would have developed space travel, and with all those options, at least one of those alien races should have "discovered" Earth, since the envelope of our electronic spectrum, showing that we are hear, now reaches beyond Alpha Centuri. Fermi's response was, "So, why are they not here?"
The anser may be found in the first law of the Federation in Star Trek, which is "non-interference." We would be to these space travelers as primative as head hunters in Boreo are to us. The simplest explanation is that they are there, they know we are here, but they are leaving us alone until we become sufficiently civilized to be invited to join the Federation.
"The simplest explanation that is consistent with the facts is most likely the correct one." Occam's Razor.
Conressman Billybob
It's not that simple; because it is most unlikely that every civilization in the galaxy plays by the same rules. All it takes is one rogue civilization to break the rules.
Nope. There are always Klingons or Romulans, who say "up yours with your sissy Prime Directive."
In other words, if intelligent ETs are common, one or more of them will refuse to play by the "U.N." rules and come calling.
--Boris