right now the Ukrainians haven’t extended so far that they have to concern themselves with a flanking maneuver. The countryside is acting as their defense.
People draw maps like this that conveniently ignore geography and available infrastructure.
There are no highways along those arrows. There is thick forest and several rivers.
Look at Google Earth for
Luk'yantsi, Ukraine.
This is near where any pincers would link up when pinching this penetration off.
There is a lot of steppe (very flat or gently rolling), a few forests, at least not one that would stop tanks. (You would be surprised at what trees a tank can drive over)
There are creeks, but no rivers, and this time of year, I bet the creeks are pretty dry.
The reason Russia is not pinching this off is the lack of troops and the inability to use HETs to move tanks that distance. Tracked vehicles are notoriously unreliable. Just driving them 110 km will cause a certain number to break en route (even if the maintenance officers have not skimmed parts off the top and sold them for private profit, which is unlikely in Russia).
As for troops to pinch this off, I would imagine that the Russians are engaged in a max effort to nudge the front forward inside Ukraine and they are scrambling for troops to stop the Ukrainian penetration into Kursk Oblast. Finding enough troops to hold the shoulders and then pinch this off will be enormously difficult. Ukraine is probably focusing its ISR efforts on the roads leading to the shoulders.