I'm not sure what the defaults are on Gboard because I've tinkered with the settings a lot by now, but you can have both long press to change language and swipe to move cursor at the same time, they don't really interfere with each other, I just did both while writing this comment.

Also I'm not sure how many people have more than one language enabled on their keyboard, I'm admittedly not in a great position to judge that as an American since not too many of us speak more than one language with any regularity or fluency. A lot of people don't tinker with the settings too much and for a lot of languages that use some variant of the Latin alphabet using the keyboard of your main language is probably sufficient for most other Latin alphabet languages in like 99% of cases. I suspect a lot of people don't bother (though I'm happy to be proven wrong)