Because the only reason people would even consider that the invasion would have been avoidable if only Ukraine had ruled out joining NATO is because Russia launched a PR campaign in advance of the invasion insisting that all it wanted was for Ukraine not to join NATO (its most effective PR strategy of the whole war)

In which case it's also worth taking into account that Russia was very loudly coming up with a bunch of other rationales for why it needed to go to war for domestic consumption, none of which would have gone away if Ukraine had ruled out joining NATO, making it even less likely that all Putin cared about was the remote possibility that it might join NATO at some unspecified time in future