Easy... just because Samsung refused to pay money to license NTFS from MSFT. When you are a small unknown company, they probably would not mind paying small fee to license NTFS (or maybe they did not license them and just use it anyway since MSFT probably wouldnt care if a small company is abusing them). Samsung sells tens of millions of phones so every cent counts.

Flagship phone so fragile? The water won't go in within 30mins unless.. they misleading by saying is IP68.

I dont know about your usage, but for me, when I was using S9+ (before warranty ended), I already feel the back cover lifting and have gaps that moved when I pressed on the area. For me, I think my usage of gaming and then fast charging 2-3 times everyday contribute to this problem. Thus now I buy FE and then have another gaming phone. For comparison, my wife S8+ (screen replaced on 15th month due to screen turning b&w) and charge every 1-2 days still have decent gap to this day. She upgraded to S23u last year.
Gaming?

S23u already 2 years, battery still good, the leather case still good.
anyone know which brand got sell light casing ? not more than 20g

After 2 years, change yellow like the others.

The other no-name clear ones I am using for a year is also still OK. So I think they are all more or less the same.