I'm not sure what the UI/UX similarity is between those websites that you're comparing, but I do know one thing : those websites are too big to radically change

I'm not sure what the UI/UX similarity is between those websites that you're comparing, but I do know one thing : those websites are too big to radically change without a lot of thoughtful time considering the outcome. 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.