I think this goes to the fact that the people designing, implementing and signing off on development are looking at the UIs constantly - close to 8 hours a day, every day.
This is usually (at least) an order of magnitude more than your users (well unless you're Facebook I guess). I think many people just get bored looking at the same old screen all the time; and so when a designer inevitably proposes something different everyone on the team thinks it looks great and so much better than the old design.
It's hard for them to realise that it's not better, but merely alleviates the boredom they feel.
That or some designers are opinionated and more interested in making their mark on a design, compared with understanding and advocating for their userbase.
Honestly I've heard this line more from designers than any other field:
"We can't add that feature/button because it's too confusing"
Usually the above statement is conflicting with reality and the request to add the feature/button is to give the UI clarity (or worse, provide the feature the customer asked for!)
This is usually (at least) an order of magnitude more than your users (well unless you're Facebook I guess). I think many people just get bored looking at the same old screen all the time; and so when a designer inevitably proposes something different everyone on the team thinks it looks great and so much better than the old design.
It's hard for them to realise that it's not better, but merely alleviates the boredom they feel.