Planes are a safer bet once you have secured half the country to take off and land from, which happened last week. They are also a better risk once you have seriously degraded the anti-air and radar capabilities of the invaders which Ukraine has.
They are getting the migs and also helicopters they want along with long ranged artillery and artillery targeting systems Ukrainians are all ready trained to operate and maintain.
It seems like Ukraine is testing Russian force protection in an expanding strategy including a main supply rail line and the flagship of the Russian Black Sea fleet along with the usual assortment of tanks, armored vehicles, weapon systems, trucks, and soldiers.