NATO is a group where everyone agreed if one of them gets punched, they all punch the bully back. Because of this, the bully doesn't punch anyone in the group. The bully could make the punches painful for NATO, but NATO could do the same to the bully.

Ukraine is being punched. If they join NATO, all the NATO states have to punch back, and then the bully punches NATO because they're already being punched.

The threat of being punched keeps NATO and the bully away from each other. If one side actually starts punching, the other side would too, and both would be punched. So NATO isn't willing to let Ukraine in and immediately start a fight