Pentagon explains why they don't want Ukraine to strike deeply into Russia. TLDR: It's because Russia is like your crazy cousin that always gets arrested and you don't know what he'll do next.
While I disagree to a point, I think that it may be more than this. Perhaps they are wanting to bleed Russia slowly so that more in total is lost in terms of men/tanks/arty. This conflict is could certainly be viewed as a proxy war between the US and Nato on one side and the Russkies on the other. If so, a slow stalemate leading to victory may be the best way to weaken Russia and its ability to project power for the next decade.
https://news.liga.net/en/politics/news/pentagon-explains-withholding-permission-for-ukraine-to-strike-deep-into-russia