Most accessibility affordances have a negligible cost when designed in from the beginning and are significantly more expensive to retrofit. (E.g. designing a staircase with an integrated ramp is cheap; adding a ramp to an existing staircase often is not.)
Ensuring that everyone has equal access to society seems like a pretty worthwhile goal and I'd hate to see people locked out of it to save a fraction of a percent in costs.
There are physical limitations to construction. Sometimes things take time to set and/or cure. Even when doing the work, it’s not like you can have five guys working on a single pipe at the same time, hence the “breaks.”
My EU aparment has an elevator, and you can get to every room in a wheelchair.
Can you get into the upstairs bedroom on an average american home in a wheelchair?
But if only 1 person was in a wheelchair it wouldn't make sense to spend trillions of dollars making the world accessible to that person.
There's tons of people with tons of disabilities.
It just so happens that there's lots of people with certain disabilities, and if we can do things that aren't too cost prohibitive to make life way easier for a lot of people than we should.
But it's still a debate about cost.
And obviously different people will have different views.