It has both by default. If you hold, you get the language selector but if you swipe, it moves the cursor. Gboard also has some acceleration on the swipes which makes them wildly imprecise, now I use florisboard which doesn't do that.

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)