That's not true. There's a couple of forests that are as dense as gameplay can reasonably allow.

Most of the environments are empty planes with a 1-2 trees I think they needed to use a lot of tricks to have more than that. It might have also been an ai pathing issue

Most of the time isn't the limitation? Maximum is the limitation. Look at the forest on the plateau. Look at where memory 11 is in BotW. Look at the lost woods.

Sure, most of the time there are just a few trees, but when they want to they blow right past that