Because it is their own product, and it could be seen as a negative move to not use it?
Using WMP Plugin, RealPlayer plugin, QuickTime, Flash, etc. is so... 2007.
MS also has(had) Silverlight but at some point they just stopped using it and most of the videos I've seen recently on their site(s) are HTML5 (no plugins required).
But not Apple. Everything (I've seen) on their site is QuickTime.
> Everything shows up as HTML5 video for me.
That's weird.