That would cost money, but how could it possibly cost as much as modifying every building everywhere? A wheelchair ramp is thousands of dollars. An elevator is hundreds of thousands. A person in a wheelchair might encounter a thousand distinct sets of stairs in a year but the median set of stairs doesn't encounter that number of distinct people in a wheelchair.
More importantly, there are stairs in places we're not going to do anything about it, like old buildings and private homes. And there are accessibility devices that become inaccessible, e.g. an elevator that can't be used because the power is out or the building is on fire. A solution that still works in those cases is better.
As far as I can tell we got stuck with the existing one because of politics. The per-person cost of a ramp at a large high volume business is negligible because it has so much traffic, so much so that they'd install one regardless. The real cost is putting them in every small shop and low traffic area. But then you have no opposition from large businesses to a law requiring them because it puts a disproportionate cost on their smaller competitors. Whereas a tax that paid for more capable "wheelchairs" would sensibly be paid mostly by those large companies, and then they object.