- The Ukrainian ground forces are saturated with MANPADs after 8 years of continuous pumping (to the point of untrained personnel getting them), which has significantly accelerated at the end of the last year. So the Russian Air Force can not operate easily on low-mid altitude far from their ground forces.
- The remaining Ukrainian anti-air missile systems (like S-300, but there are also older ones) are heavily supported by NATO radar infrastructure in Poland, so it's risky for Russian planes to stay long on high altitudes.
- The Kalibr missile is quite sufficient for striking stationary objects, so you don't need planes for those. It's hard to strike mobile forces, because they quickly get intelligence from NATO about planes in the area and thus able to hide in cities, thus making it hard for RAF to strike them (the Russian forces have enough firepower to level cities to the ground, but they have orders to minimize damage if possible).
- Air supremacy is already achieved (almost all Ukrainian war planes and strike drones have been shot down), so it does not make much sense to patrol the air space too much, since operating a plane is far from being cheap.