Because we have the capability to work beyond that capacity now in most cases. That's like asking "why shouldn't we allow horses on our highways?"
> Pretty much everything I consume online is plain text, even if it happens to be styled with images and fancy javascript.
No doubt, pretty much everyone who works on web apps for long enough understands that it's total madness. The cost however, in supporting people so far behind as to only be able to serve them text is quite frankly unmanageable. The web has grown dramatically over the past 20 years both in terms of physical scale and supported media types.
The web is becoming a platform delivery service for complex applications. Some people like to think of the web as just hyper text, and everything on it should be human parse-able. For me, as someone who has come late to the game, it has never seemed that way. The web is where I go to do things: work, learn, consume, watch, play. It's a tool that allows me to access the interfaces I use in my daily life. I think there's a ton of value in this, perhaps more than as a platform for simple reading news and blogs.
I look forward to WebAssembly and other advancements that allow us to treat the web as we once treated desktop environments, at the expense of human readability. It doesn't mean we need to abandon older + simpler protocols, because they too serve a purpose. But to stop technological advancement in order to appease the lowest common denominator seems silly to me.