Someone correct me if I'm wrong, but I think <embed> was used because it embedded a flash player, back when flash existed on the web. Then browsers decided to get rid of flash, and something had to replace it. You can't <embed> HTML pages, so wasn't really up to YouTube if to break it or not.
16 people say I'm right, one person say I'm wrong. I'm not doubting you, but I'd love it if you could show how I'm wrong? Maybe I misremember how to use <embed>.
Where? To be clear, it doesn't matter—being wrong is wrong. Those other people, even if they exist, are wrong, too.
(I can half-understand not checking before you posted the original comment. But why wouldn't you, in response to someone letting you know that you are wrong—which was itself in response to you asking, by the way—take half as much time to just check as the amount of time it took to type out such an bewilderingly obnoxious followup?)
Youtube didn't change - browsers did. Embed loads plugin content: browsers broadly moved away from plugins (they'll still technically work if you feel like jumping through installation hoops as a user but they're not maintained and not broadly preinstalled as they used to be).
During this broad migration by the browser community away from supporting plugins within embed tags, there was likely some audits of how widely they were relied upon across the web, and compat hacks put in place to account for the most common uses.
It's very likely Youtube removed endpoints after the browser hacks were in place for some time.
It’s pragmatic. End users don’t care enough to place the blame correctly. All they think is “my iPhone can’t play this video that my friend sent me, my phone sucks”. It just doesn’t matter whose fault it is that that’s the reality.