I am not sure that I understand your point? DRM has not been a part of the HTML standard, that is true. But because of this, I remember having to install Microsoft Silverlight to watch Netflix. I do not really use Amazon video unless it is on my Roku, so I do not know what they used.
Simply stated, movie studios do not want people to stream movies unless DRM is involved. It just makes sense to have the DRM be as simple and unobtrusive as possible.
I wish that we COULD live in a world without DRM. I do not buy games with burdensome DRM. I avoid Electronic Arts because of the bloody abortion that they call "Origin" (I tolerated it enough to finish Mass Effect 3 and then ripped it out of my computer). I do own some stuff that uses Steam, but that is as far as I will go. Generally, I get my games from GOG.com or Humble Bundles (prefer the ones that are DRM-free). When I buy e-books, I try to get DRM-free also (O'reilly books are awesome for technical stuff). However, going completely DRM-free is impractical unless you are willing to not watch any blockbuster movies in the same decade that they are made. Even DVDs have DRM (although it is trivial to crack).