Because modern air war has little if anything to do with dogfighting (planes fighting planes with guns). It is all about using missiles to hit targets. Both on the ground and in the air. And those missiles can also be fired from the ground. SAMs (surface-to-air-missile) are used to hit planes, drones, other missiles. Also MANPADs (portable shoulder fired missiles) pose a threat to aviation.

In an air superiority battle overwhelming air power is used to take out the SAM batteries. This is difficult and costly. Something neither Russia nor Ukraine are currently able to do. So both are using jets in a minimal capacity.

Recently Russia has ramped up the usage of jets to hit entrenched targets on the front line. Which has predictably resulted greater losses. And relatively meager gains.