The route convoy took is shielded by bodies of water on both...

The route convoy took is shielded by bodies of water on both sides.

Said convoy

The the east of the convoy is the river Dnieper, which at this point in the flow is almost a lake, not a proper river. To the west are marshes and several smaller rivers that flow through them. The convoy is long, but protected from the sides by geography.

It’s also protected from the above by Russian air power and integrated air defences. Ukrainian air force has most likely been ground into nothingness by now. Nothing has been said about it for several days, the most likely reason for that is they’ve lost operational capability entirely.

Ukrainians might be able to stage infantry assaults through the marches, but it would be a bloody exercise. Russians aren’t as foolish as not to defend it with infantry of their own you see. Ukraine most likely lacks the ability to strike the convoy effectively, that’s why it’s still there, seemingly unmolested.