Obviously there are many ways to do it (third party web services, plugins, etc.), but I guess the outcome is a bigger attack vector for users that download questionable stuff to do it anyway.
Youtube is constantly on the knife-edge of being sued en masse by large media owners, as they were by Viacom. It's why their copyright strike system seems "broken," when it's really operating as intended, giving copyright holders arbitrary power to remove content they feel is infringing (without regards to fair use) because anything less would probably lead to those companies to simply prefer to sue Youtube into oblivion.<p>Technically speaking, of course Youtube could have a download button, but if they did, as far as the Viacoms of the world are concerned, they're just enabling piracy, and taking control away from content owners.
<a href="https://support.google.com/youtube/answer/6308116?hl=en" rel="nofollow">https://support.google.com/youtube/answer/6308116?hl=en</a><p><i>Download videos and playlists to watch offline for up to 30 days when you aren’t connected to the internet.</i>
Because musicians still get paid by the sale or play (like radio).<p>It was far cheaper to treat streaming like radio, so YouTube scans uploads for music and pays money to musicians for each play.<p>If youtube encouraged downloading, there would be no easy way to count the number of plays of music or pay musicians, which would lead to lawsuits galore.
Aside from what is already mentioned in other posts, there would be logistic concerns to enable downloading in a large scale.<p>By assuming the content will be watched by a human, they can throttle buffering to transfer only what's needed for human consumption, thus spreading out the network load.