In addition to the imperialist and Israeli lobby reasons others have outlined, there are powerful factions within the US that actually want this war to go really, really badly.
The totally insane but maybe less influential one is the Evangelical rapture segment. If I understand correctly, they want there to be a violent cataclysm in the holy land to usher in their heretical fantasy of the end times. Absolute death cult, no idea where to best read about them, but they are a significant part of Trump's Heritage Foundation sponsored voter base. Apparently Hegseth recently had commanders tell the deployed troops some insane drivel along those lines.
The other faction are the Silicon Valley billionaire mafia, like Peter Thiel, Marc Andreesen, Elon Musk and their ilk. They are pretty open about their plans to shatter the nation state into small dictatorships, as laid out by their court philosophers like Curtis Yarvin and Balaji Srinivasan. Look into their books. If the media talks about the tech barons, it's either glazing them as AI pioneers or painting them as greedy war profiteers. But outside a bunch of long nerdy think pieces, there doesn't seem to be much discussion about their coordinated campaign to destroy the state, for which a disastrous long war in Iran is the ideal development.