Well, the short answer is probably that the West, in particular the Biden administration, asked them not to. They _have_ done it a few times, but rumor is that

Well, the short answer is probably that the West, in particular the Biden administration, asked them not to. They _have_ done it a few times, but rumor is that they get western approval first. Why do they care what we think? Well, because we're supplying them with weapons. Since everyone knows this is now a proxy war, similar to when the Soviet Union was supporting North Vietnam or Chinese "volunteers" came across the border to support North Korea, not striking at Russia (too often) is a way to attempt to avoid escalation. Also, the primary objective is to eject the Russian troops from Ukrainian soil, so there's not much reason to attack Russian territory most of the time. Lastly, another reason why Ukraine is currently beating a much larger opponent is that the Russian troops are said to be not especially motivated, in most cases. If Ukraine attacked Russian playgrounds and apartment buildings, as a tit-for-tat, it might even motivate their (currently not especially motivated) opponent. Ukraine is actually concentrating on things that advance their military objectives, and attacking Russian territory targets usually is not the best way of doing that.