Putin wants all of Ukraine, Ukraine wants Putin’s army to fuck off.

If they stop all advancing that will allow Ukraine to hold a static position and establish themselves, that will put them in a stronger position to launch attacks against Putin’s army. So Putin needs to keep pushing at the Ukrainian line to keep their established positions as far back as possible. Keeping the established line further back is why Ukraine needs longer range capabilities, to extend their reach to Putin’s established lines.

By performing attempts at driving into the Ukrainian line, it provokes Ukraine to direct resources to that point. Ukrainian forces drawn to one area weakens the line elsewhere so Putin’s forces can hit another area with the hopes of breaking through and setup for flanking the adjacent Ukrainian line and thereby carve out more land.

The Russian military commanders want to make Putin happy, the only way to do that is to make progress in capturing Ukrainian land. So they must keep throwing their troops into the Ukrainian fronts; otherwise they will only lose ground and that will make Putin unhappy, when Putin is unhappy windows get broken.

So Putin’s army staying put causes their military commanders to have accidents and allows Ukraine to have a greater tactical advantage.