I'm not sure why you think the UAF can't do that. They've taken other territory and have shown a capacity for offense. A brief look at the map will show that

I'm not sure why you think the UAF can't do that. They've taken other territory and have shown a capacity for offense. A brief look at the map will show that holding Kherson is basically impossible and a huge risk for Russia. It has a single bridge across the Dneiper and it's on the outskirts of the city. That's the only supply/retreat line forces defending the city have. There's no good options. Defend the city then Ukraine attacks the supply line. Defend the supply line then Ukraine attacks the city. Try to do both and risk losing everything.