The west is stopping Ukraine from doing this for their own good.
The west wants to drag out the war to bleed out Russia.
Seriously Striking Moscow (for example) would almost certainly result in a swift, all out retaliation from Russia towards Ukraine.
The western media might not like to portray it as such, but this is still largely an excursion using older equipment and avoiding mass mobilization, preferring to send “endesirables” like criminals or the poor, for Russia, while Ukraine has been running out of equipment and men.
This is a case of “fog of war” or not seeing the forest for the trees with Ukraine, the west isn’t as biased in their analysis of the situation.