That's intentional on YouTubes end, they aim to serve more bitrate-efficient codecs wherever possible, even if it's a high burden on the client due to a lack of hardware acceleration. They'll only fall back to older codecs if the client is completely incapable of handling the modern ones. It's annoying but at their scale it no doubt saves them a shitload of bandwidth.

Classic externality: at their scale, the power costs offloaded to their clients will also be enormous.

It also encourages users to upgrade to newer hardware since older devices are known to get slower as they age due to software increasing complexity and hardware mitigations (yes, they are also for phones). Most users will just blame the device.

Not saying that is the cause of this slow down, but since the mpeg4 patents don't expire till 2027(?) (and one of those patents prevents hardware decode on Linux) we as a society have given Google every incentive to do this and I welcome them to make mpeg4 irrelevant.