That's because 15.0 is missing the key "features" they showed off for it (all the FaceTime stuff isn't working yet) and I'm sure they don't want people to install it and then class action over the lack of anything actually new in the new update.The software update screen was telling me that my “iOS is up to date” at 14.8. Then a small banner at the bottom told me I could upgrade to iOS 15 too. Apple doesn’t want people to update.
I showed Live Text to some of my family members and friends - all of them updated to iOS 15 immediately afterwards.
I'd hide that it was available too if I were Apple.