Apple's iOS updates coincide with new product releases, which in this case was the iPad Air and Apple Watch 6 and SE. They have to have something ready for those product releases, since they can't ship without an operating system.
On past releases, the XX.0.1 or XX.1 beta tests were already underway before the initial XX.0 release comes out. Apple is already beta testing iOS 14.2, which presumably will come out at the same time as the iPhone 12.
The only showstopping bug that Apple had to address immediately was iOS 10.0. IIRC, that update bricked devices that used the OTA update by sending it into a recovery mode loop. They reissued the update along with a bug patch hours later as 10.0.1. As I've said before, I think on major updates especially, people should use the Finder or iTunes to do the update if they can, because that will ensure that their device replaces the entire iOS contents. The OTA updates only replace the delta (changed) files, and that's where random glitches can accumulate over time.