Now, I'm no UI guru, but it's pretty clear that these platforms seem to have crammed in a boatload of info without much consideration for user-friendliness.
So, what's the deal with that?
Well, each of those companies has a lot of people saying the same thing against a tide of people that say "Well, it's working now, and changing stuff; that's a risk."
The same struggle that exists anywhere with decisions made by a group of people.
So, my answer condensed : Internal friction and fear of poor outcome prevents large companies from 'rocking the boat' more often.
Engelbart said:
"…if we were focused on making everything easy to learn, rather than easy to use, we would all be riding tricycles. The bicycle is harder to learn to ride, but much more powerful."
I've never used Steam's site and haven't used Amazon in quite a few years so I can't comment on what their designs are like at the moment. But I do appreciate the no-fills aesthetic of Craigslist. Always have.