All four examples requires web browser to download additional software in a form of scripts bundled into the website. And three of them all propietary apps, so it’s better to refure their use.
Don’t really know why they build them this way, browsers can send and recieve files without it via POST and GET methods, why requiering additional JavaScript?
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.