Why can't embedded YouTube videos be maximised?

Sorry for cross-posting, but I posted this thread in the "Administrative Stuff" section and it hasn't had any replies (is that forum visible to everyone?).

https://www.adisc.org/forum/showthread.php/106731-Enable-full-screen-embedded-videos

Anyway, I notice that you can't maximise embedded YouTube videos on forum posts. I'm sure this used to be possible. Is there any reason for this? Does anyone know how to thwart the will of artificial intelligence and make it obey my commands to maximise at once?!

I have this problem in other forums as well - it's simply because the embed plugin is buggy. It never really works very well, and they often remove this functionality entirely, just due to bugs with it.