Tell that to the Grand Army in 1805, when Napoleon took 200,000 troops on a rapid forced march and trapped the Austrians at Ulm while bypassing almost everything. The Duke of Marlborough fought 30 sieges and like 4 pitched battles, simply because in the 18th century the average army size was too small to bypass major cities and fortresses. Doing so would leave an army's supply lines and communication lines badly exposed, as you said. But the French Revolution and the rise of mass conscription changed all that. France could suddenly put an army on the field larger than that of the Roman Empire, despite a much smaller population. That's why sieges declined in importance during the Napoleonic Wars. The French simply masked big targets with residual forces and continued on their way to destroy the enemy's field army. That has been the basic model ever since, but it hinges on a critical detail: you need a large army to pull it off, and some mobility in the war theater so you're not stuck in the trenches.
And that brings us to this war. The central problem with what the Russians did in this war isn't that they failed to capture places like Sumy and Chernihiv. It's that they failed to capture these places and kept advancing with a small army. They didn't bring enough troops for the kind of war theater they were facing. If they'd gone in with half a million troops, they could have easily encircled places like Sumy and Chernihiv and just bombed or starved them to death. Then there would still be enough troops in the rest of the Russian force to destroy the Ukrainian field army. Now they're stuck with the worst of all worlds: they haven't captured these cities and their supply lines are getting wrecked because they don't have enough troops to properly protect them. Also important to emphasize that this conclusion is very theater-dependent. Russian control in the eastern and southern territories is much more robust. Their stuff isn't getting raided like crazy over there. It's just the northern theater that has been catastrophic.