That Russia sent in such a rich sitting duck target and it's not being attacked suggests they have air superiority and we're getting bad information.
Obviously the first thing that pops to mind when you hear “air superiority” and “40 mile convoy of enemy tanks” is “bomb that convoy to dust”. The second thing that should pop is “ambush”. Losing 5 aircraft to surface to air missiles to take out 5 tanks would be a poor tradeoff.
Doubtless bombing the convoy is something Ukraine has considered. Also doubtless they are aware that the convoy isn’t guaranteed to be a parade of sitting ducks.
Maybe they didn't bomb the convoy, but just the supply, like they did before.
Once those soldiers go searching for food, the Ukrainians can take some extra tanks for their army ;).