Because the plugins in question are for the operating system's video infrastructure (Quicktime and DirectShow), not the browser.

<video> still acts as a standard platform API across browsers regardless of codec support. It's not like the bad old days where you would have your page manually loading opaque QT or WM or Real plugins through <object> and <embed>.

as mentioned in the post, since firefox is 25%+ of the market and will never budge, it was alway going to have to be webm or plugins.

no one (hopefully) wanted this situation, but the reality is that the different parties involved disagree strongly about what codec should be the defacto standard.

Because Apple and Microsoft are able to obsolete these plugins if they choose so (I'm not saying it's the best solution, but it's legally and technically feasible), and others don't require those plugins.

It's not that easy the other way, where you've got closed reference implementation of Flash (and you can't get it without plugin) and there are patent royalty payments for H.264 decoders.