Why does it need to be the U.S.?
Why can’t Poland use another NATO and/or EU partner as a proxy? Or, better yet, provide them directly?
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?