Because IIRC as part of the NATO charter, sending those aircraft directly to a warring party outside of NATO without the consent of the rest of NATO means that if they are attacked the rest of NATO does *not* have to come help.
Or put another way - if Poland had provided aircraft to the Ukranians directly as Blinken had been saying he wanted them to and Russia struck back, Poland could not expect the rest of NATO to help them. They were understandably uninterested.
More specifically, do you really think our current administration would have gone to defend Poland if there were Russian reprisal strikes? Or would they have suddenly ‘discovered’ the clause and ‘regretfully’ told Poland they were on their own?
That would surprise the Europeans because we asked for AWACS under Article V after 9/11.
So could they have said we were SOL because 9/11 was blowback for our involvement in the Middle East?