You also need the complete supply and support line available. Also, the 'donated' F16s first need to be freed (replaced by F35s) or fixed (some have parts removed to be used as spares for others still flying. Both donating countries still have them in service. Denmark is waiting for their 1st F35 to be delivered, The Netherlands has them still in active service for air patrol/rapid response (and only a few F35s delivered).

Also, it's not enough for Ukraine to allow foreign pilots to fly for them, the countries that they're from need to allow them to go as well (as they usually paid for the training). Planes are expensive, but not so expensive as the pilot. (and it takes decades to grow and train new pilots ;) )