What air defense ~~doing~~?

Sleeping

The dumb little thief used most of Russia's massive AA missile stockpile to commit war crimes (bombard civilians) after they ran low of their actual missiles (because they bombarded the civilians so much). They simply haven't got that many left. Also, Ukrainians got really good at finding and destroying the launchers. They have few missiles and fewer launchers still left. The few that are left are in use in the warzone, not 600km behind it.

Source: armchair general with zero actual military knowledge

I think they're busy not protecting the boats in the Black sea. It takes a lot of effort to not protect those boats, and they haven't got any spare personnel left over to not protect aircraft.

Meanwhile the soldiers on the ground will have to not protect themselves.

Hell if you're going to be conscripted by Russia then your best help is to get conscripted into air defence because it apparently doesn't involve doing anything.