Just because the Russian army doesn't have air superiority doesn't mean they have no air support at all.

It's possible the convoy is protected but that the Russians can't or don't want to deploy more aircrafts for other operations.

It's also possible that the Ukrainians don't want to attack the convoy but the support this convoy needs to get. That's a powerful tactic if you can cut off the supply chain for such a massive number of vehicles.

Note: I'm no military expert (is there a handy acronym for that?) :-)