It’s Called “decency”
guessing you never heard of that.
WWDC is the announcement.
If Apple had announced it would be software-only, the only rumors that would have stopped was regarding new MacBooks.
Big software changes are exactly what a lot were expecting for iPadOS so unless Apple pretty much releases all the details prior to WWDC, that wouldn't have stopped the disappointment for those with high expectations for the M1 iPad.
And really, they've already said they weren't merging MacOS and iPadOS before WWDC. Not exactly their fault when people chose to ignore them.