Publishing is how scientists get their street cred. Thus, the scientists themselves want to publish in big name journals to up their rep - hitting something like Nature is major coup. And then they can convert their standing in the big science gang to things like research grants, commercial projects, academic tenures, etc.

If you don't care about how science street cred works, nothing stops you from just throwing your papers up on arxiv. But then you get no publishing rep. And no visibility either. A big name journal in a given field gets eyes on your paper by default - but in the pits of arxiv, if you don't put your work out there yourself in the circles, no one will see it.