The general answer is that this timer is simple, elegant, and versatile. When the game creation interface allows you to set the timer not with numbers, but with buttons like Blitz/Rapid/Classic, the numbers will be exactly like this: 8+4+1/12+6+1/16+8+1. No one will create buttons like 21+8:30+1:15, because these numbers are not aesthetically pleasing; it's better to just use 20+10+1.
I can't imagine why 16+8+1 is considered "extremely short," while 21+8:30+1 is considered "just right." Why not 24+9+1 then? The arguments would be exactly the same.
Yes, it's more difficult to play perfectly on a mirrored map with the same timer compared to, for example, Jebel or HLU. But the point isn't to play perfectly, but to play better than your opponent, taking into account all factors, including the timer.
So, let's not bring up this issue again and again. There are as many opinions as there are people; we need to settle on something. These extra 30 seconds per turn won't globally change anything.