There's the points others have made about the business model - for a long time, the "momentum" oriented approach was essentially a Ponzi scheme where investors would invest in a business that would take the risk of major losses so that they could destroy all competition in a space, then eventually, turn a profit by changing their tactics in user-unfriendly ways long down the line since you have the monopoly.

For this particular issue, though, I think we're seeing the Rotten Tomatoes effect en masse. If you want to make something bold and impressive, you need something people love or hate - not something between. With Rotten Tomatoes as an example, it's binary - Positive or Negative. This incentivizes movie production to produce things that are not controversial, just things that people won't strongly dislike.

With centralized platforms, the product models stopped being about providing high quality products and began valuing time spent on the platform. Produce a website/platform that most people are okay with and the majority aren't extremely opposed to. This means it won't do anything bold, but it does mean you'll pick up a critical mass and become the dominant force, as you're appealing to the majority.

In a content-driven economy, whoever has the users and the content rides that positive feedback loop to monopolies. More users = more content = more users.

Algorithms get worse because they're appealing to "Good Enough". If it gets bold and suggests something that you might either love or hate, then you might hate it and leave the site for a bit, but if everything is good enough, you'll stick around. Web design gets blander because things get familiar, and especially after the start of Facebook, we learned that people really choose familiarity over novelty. Movies, TV, and Music get blander because they are now driven by the same platform economics where sticking around on the platform is valued more than appreciating the content of the platform.