F16s are notorious for needing very special conditions in the airfields to operate. They have neither the time nor the resources to make and maintain the airfields domestically in Ukraine. But the NATO countries have the budget assigned and the places far behind the front lines to be able to do it. If they try to do so inside Ukraine, they risk being targeted by Russian missiles and air force, specially now that the Ukrainian air defence is basically depleted. Using NATO bases is not just an adventure, it is necessary given the conditions to which the AFU has been degraded after two years of war. Two years ago, that was maybe realistic, but Russia made sure to strike airports and hangas non stop at the start of the war. There's a reason we have seen less and less Ukrainian planes as time passes, and more and more Russian ones.