You somewhat answer yourself in the question: "what is keeping you from volunteering...?"
The need to volunteer is. If I devoted my life to volunteering my family would starve and be forced to live on the streets.
To ask a more reasonable question, why companies aren't investing more into accessible interfaces, it comes down to numbers. As many have pointed out, you're talking about a small fraction of the userbase who can't use the typical keyboard and mouse setup. But even beyond that, you don't even define "disabled". There are countless types of disabilities, each requiring specialized adaptations. You can't simply build an interface "for the disabled" because you first need to know how a user is disabled. This means that, in many cases, you're talking about a customized device. That's rarely cheap, even if insurance is involved. If the customer isn't willing or able to pay for it, nobody's going to make it.