Only because they've chosen not to

If you had a big corpo being affected by every knife murder, you'd be sure that they would be liable. Since murders usually "only" affect an individual nothing is being done

First, courts do hold manufacturers responsible for manufacturing or design defects.

Then, generally, "software is just a tool like a knife" doesn't hold because software today is more a service than a tangible good. Companies continue to update the programs and things can happen in the cloud with limited user control.