And the Ukrainians have destroyed an untold number of Russian ammo dumps in the last weeks. Some of the explosions have been truly spectacular. This isn't something you can easily remedy. Any new ammo dump in range will be quickly spotted by either American spy satellites or local Ukrainian spies that are probably still active in Donbass - and get blasted to hell.
HIMARS is not enough to reconquer the lost territories, true. But it is enough to give Russians a forced operational pause.