My question is why shouldn't companies be liable for how their product harms people?
Because "harm" is not a binary condition, nor is "liable."
Regulation shapes commerce. Fettered capitalism isn't always better than unfettered capitalism. It's entirely rational to argue about the fetters. I personally believe that corporations should be more responsible for the externalities of their business models but I also know that innovation often races ahead of morality and then there's a correction.
If every website policed their own content we wouldn't have bills like this but since they don't, the ones that ignore it make that money and drive out the ones that do. Textbook tragedy of the commons and before too long there's arguments about how much we want the sheriff to control grazing.