Comment by charcircuit

Because the platform is too hard. The average person does not want to spend time figuring out how to get stuff to work. All the browser documentation is 1000 times nerdier than the average person wants. They want all this low level stuff abstracted away from them.

Even if you want to do it the nerdy way and bust out a text editor and start writing some HTML tags it all looks ugly out of the box. Browsers pushed everything for making websites onto 3rd party devs, so they shouldn't be surprised when they want to use 3rd party dev's stuff over the bad and confusing 1st party ones. The browser developers are in an ivory tower building a product that doesn't really care about web developers and their needs.