For a lot of these websites it would necessitate a lot of manual work, the unfortunate reality is that although producing accessibility features is not 'hard' it can be time consuming and most corporates won't go for it.
Yes, but ... That's work they brought on themselves by not doing the right things in the easy way. Sure, once they're there, there is a lot of work to come back but that should be used as a reason to not use those frameworks in the first place.
Some accessibility is like adding alt-text. It adds work. But almost everything else is just not abusing the browser and is generally done simply by following the recommendations. And not trying to live on the bleeding edge.