Apparently, copyright law makes this an unwise proposition. But the least they could do is offer the Internet Archive with some privileged access so all this can be mirrored and viewed when it's out of copyright in 80 years.
They could at least let you download videos for which the author specified a free license, such as Creative Commons, or videos that are public domain. There are a lot of these videos so why not?
Just discovered that several video sharing sites, like pexels.com, have a download button on videos with creative commons license. They are off course useless for professional YouTubers, but great for everybody else.
It's not copyright law, it's the agreements that YT has with copyright owners. No one is going to license their content to YT if the site allows anyone to download whatever is there.
If it wasn't for copyright law, there wouldn't be copyright owners. So copyright law is ultimately responsible for the status quo.