There's a lot of complexity here that you're overlooking. First of all, the sites have KYC and require geolocation software so it's not trivial to play. Especially not for the bot developers which have tended to live in Eastern Europe or Central Asia. They'll just go wherever is easiest to make money so it doesn't necessarily follow that every site would be overrun.
Second of all, poker is fairly capital intensive and whenever your bot account gets banned the site will confiscate your funds, so there's risk involved as well. And every time you get banned you need to create a new account with new KYC etc.
Third of all, bots play differently from humans and many of them are detected and caught by the players in addition to the site security. Further adding to the challenge is that the community of professional online players in the US is pretty small and everyone pretty much knows everyone else (we're all on Discord together, basically). So new names appearing at high stakes out of nowhere get scrutinized more.
Fourth, even if you're playing against a bot or cheater, you can still make money, since winrate is entirely driven by fish. You might lose a little against the cheater but as long as you're winning far more from the fish you'll still make money. This separates poker from other competitive games.
I don't mean to imply the bot and cheating issues don't exist, they're real and serious and existential, and every online pro these days spends a lot of time worrying about it, worrying if a certain opponent is cheating, etc. But I think the bigger issues facing online poker are actually regulatory (in the US, an unregulated market has sprung up since the pandemic that is now struggling with a lot of legal changes; Europe has a lot of anti gambling laws these days and more every year) as well as general game quality (fewer recreational players wanting to gamble large amounts of money online and more pros than ever trying to split that smaller pie).