> there isn't really a good reason as to why a user can't block out entire subreddits

You can-ish. At least on old reddit's /r/all page. There's a block list you can curate for yourself (when logged in) on the right hand sidebar. Sadly, it doesn't work for "popular". The filter list does carry over to the new interface, FWIW.

"Displaying content from /r/all, except the following subreddits:"