Why do streaming services, like Netflix, look so terrible on browser?

Yeah so i've recently begun collecting Blu-rays again, because i've gotten back into movies, and of course i've always known that streaming doesn't look or sound nearly as good as it does on Blu-ray (UHD aswell), i know it has something to do with the amount of bitrate the service uses to stream it to you, so even though it says "4K" it's never actually 4k because of the low bitrate.

So i've been trying to circumvent this issue, then realized it looks so much better when using an app, like on your phone, ipad, smart-tv, but on browser? Yeah pretty much all the streaming services look so ugly and closer to 480p than 4k, even though im paying for the premium package thats supposed to give me that, so why only on browser? genuinely what.

And the reason im posting this now, is because I just watched a movie on HBO Max, it looked as grimey as it always does, then afterwards went to watch youtube, changed the video I was watching to 4k, then it clicked… WHY CAN I WATCH IT ON YOUTUBE PERFECTLY FINE, BUT THE PAID SERVICE IT LOOKS SO BAD??!?!?!

Of course its still not to the level of having a physical disc, its still being streamed right, but the bitrate isnt as low as to make my eyes squint, as it sometimes is on Hbo max or disney plus, why does youtube look so good in comparison, when all this time I had just accepted it as fact that browsers just inherintly look bad when streaming, when youtube has showed that that's not the case at all.

EDIT: After reading some of your answers, these companies genuinely make pirating look so much easier, because seriously, why am i even paying to watch movies in 720p in 2025…