If they do that then Ukraine can target targets deep in Russia and that will escalate and cause Russia to use tactical nuclear weapons. The US don't want escalation. They want Ukraine to be able to defend itself, not attack Russia. That's why they tweeked HIMARS so the computer cannot fire into Russia.
We need to continue helping Ukraine. I think we should move up Ukraine's ability to attack more Russian military bases and Putin called the game they should also start attacking Putin's electrical grid. Ukraine should start making Russian people suffer for Putin's war. Take out more Russian military in Russia the people will understand war better.