It’s due to straightforward human incentives.

These companies have employees. They want to keep their jobs, and some will also want to gain more money or status through recognition of their efforts. So what do they do? They come up with new features.

This can work just fine in small teams without competition between those tasked with coming up with those new feature. The worst examples are due to an org having too many people tasked with coming up with product changes, which creates incentives that result in a worse user experience, like you’re describing.