Why not? They support HMTL5/MP4 video today, which works with basically all Webkit-based browsers. It's all about reaching an audience.
As I've noted above, YouTube does more than just dumping a video onto the user's screen.
From YouTube themselves:
http://apiblog.youtube.com/2010/06/flash-and-html5-tag.html
WebKit does not support any specific video format. It's up to the individual browser to do that.
Safari is the only WebKit browser with full support for h.264 and MPEG4.
Chrome is phasing it out because it cannot be put into the open-source variant of Chrome (Chromium).