Mostly because of the land grant universities and their particular requirements, which u/DBHT14 discusses some of here and I discuss some of that as well as their funding and expansion here.

For more detail on the names in particular, /u/DBHT14 talks about it in a previous thread here, /u/yodatsracist talks about the separation between systems here, and I talk a little bit about the origination of the American university system here.