Websites for file transfer without downloading additional software or apps

probably because there's not way to police who connects to your computer in a built in way for novice users. BT just opens up your computer to anyone who wants to connect, to the seed of course.

Also, more people care about streaming on demand than file ownership/transfer these days, but content creators want guaranteed content delivery via a CDN, I assume out of some kind of sales pitch these companies do. Compared to maybe some uptime on BT, its a hard sell and I doubt BT pushers are salesmen anyways.