Why should they ? YouTube is in the streaming business, not the archival business. If someone wants to make their videos available for download they should post a link on their website, served through their servers and bandwidth.
Youtube benefits from being everyone's go-to site for finding video content. Keeping that monopoly of mindshare is worth far more to them than any short sighted profit maximisation.
Yeah that's not some kind of general rule. This is because the US legal system in particular lets this happen. And in fact, the US legal system works so badly that it often still benefits companies long after they've monopolized too much, causing all sorts of damage to consumers and other companies that actually did nothing wrong.
Isn't streaming live video? It seems to me that youtube is exactly in archival video and hardly in streaming. But it's a big site so maybe a lot of people use it in ways I don't know.
Streaming is not exclusively live video. Streaming is viewing the file as it downloads. As a stream. The alternative is what we did in the dark ages. Download the entire file before beginning playback.
WMVs can embed instructions on where to grab codecs or authorization to play the content in the multimedia. Back in the day if you played WMV with windows media player this could be used to pwn your computer.