Actually that's not always the case. In many instances, designing with accessibility in mind increases your customer base and drives more revenue. It's actually a decent size "market" which makes designing for/implementing with accessibility in mind an investment instead of a burden.
I've always found it funny when people propose some new red tape or requirement and then say "oh, it'll good for business!". Of course, if that were actually the case, the red tape wouldn't be necessary.
I've always found it funny that most people don't realize that anything more than 3 months out is "too long term to worry about" in the business world.
And yes, many large scale end-user facing app teams make a business decision to implement their UIs with accessibility in mind. For web apps specifically it's not that big of a deal these days with screen readers getting reasonably sophisticated making the additional dev effort negligible for medium-large projects.