Everybody is missing an obvious cause for the UI “sameness”: UI/UX is a discipline that optimizes for certain features, typically user engagement and ease of use. Once the optimal patterns are found, they will be implemented across the board.

For comparison, every nation on the planet has very, very similar traffic laws and signage/symbology. The optimal way to control traffic has been found, so everybody uses it. Imagine if every country had their own unique signage, road markings, right of way rules, etc. You would have to study and relearn everything every time you drove somewhere new.

Furthermore, nobody can explain why having UI “sameness” is a problem at all. Why should UI theory be re-invented from scratch for every app and website? Isn’t it better for them all to use a common visual language so users can jump into them intuitively and focus on getting tasks done rather than learning thousands of different UIs?