I am not a fan of curved screens but this is what you're stuck with if you want any flagship nowadays.
I believe the Galaxy 21 + does as well.
If I had a choice, I'd probably go flat too. But the curve doesn't seem bad enough on it's own to get the vanilla 6 instead.
I believe the S21 series; other than the Ultra, have flat displays. The ultra display looks more curved when I was playing with the Ultra and Pixel 6 Pro in the Verizon store two weeks ago. However, I did not measure to see; just from appearance and memory. Neither of them come close to the curve screens used on older Samsung phones for sure.