That's part of it - they don't want to lose their investment. The other part is that if they do stop propping up Israel, they know another country like Russia or China will happily fill the void and reap all the benefits of the influence in the region. The US could cut ties and not be involved at all but it's not likely it would change much, unfortunately; Israel would just continue with new backers and the US would be in a worse position.
Not that that justifies the war and all the weapon sales on the US' part, of course. But I think, with that information, it's easy to see why there's some nuance to it, and why the US isn't keen to sever ties or be too harsh a critic.
PugJesus