Maybe they aren't there because there is no Superman, he could have brought Earth-38 to the rest of that universe's attention, and without him there, nobody offworld, other than the Dominators, knows Earth-1 one exists, or at least cares enough to go there.
Except Superman's only been active there for 15-20 years, and we know there are aliens who have lived there far longer. J'onn has been here for centuries. I'm pretty sure Olivia Marsdin said she immigrated from Durla as a child, so that's probably over 50-60 years. And there was Amadei, that alien healer in "Parasite Lost," who'd had a baby with a human woman 20 years ago and had apparently already been living here for some time (though probably not as an "out" alien).
Incidentally, I'd been expecting the Arrowverse Wiki to merge with the 1990 Flash Wiki now, but it looks like there's been no effort to do that yet. I guess, strictly speaking, it was never made entirely explicit that the Barry Allen of Earth-90 was the exact same character from that show, rather than just someone who looks the same and has the same costume. After all, he didn't have time to reveal much about himself. But then, we've had just as little in the way of direct callbacks to Constantine, and that's been integrated into the Arrowverse Wiki. So maybe it's just a matter of time. It looks like they're still debating it on the talk pages.