It's the country next door, but it's at war, and one of the combatants is more than a bit paranoid and xenophobic, and has openly threatened NATO with nuclear escalation. Some subtypes of the MiG-29 are nuclear capable. It's top speed is at least Mach 2, roughly 1500 mph. If MiG-29s flew from NATO, Russian forces in Belarus would literally have only a minute or two to decide whether to use tactical nuclear weapons to counter a battlefield nuclear strike. Considering how incompetent Russian commanders have been so far, I wouldn't want to give them that decision to make.