Same here. And for that matter, all DAWs I know also do not use standard platform widgets. Logic Pro, Reaper, Ableton, Pro Tools, Cubase etc. all use their own custom UI elements.
You're right about the DAWs, though last time I checked, REAPER on Windows uses some standard Win32 controls in its dialogs, which helps. Also, REAPER has rich enough plugin support that a couple of plugins have been written to hack screen reader support into REAPER. In particular, check out the open-source OSARA [1] project. This work-around isn't feasible for making plugins accessible, since those plugins are generally not as hackable as REAPER itself. I don't know about other DAWs.
You spend a minuscule amount of time with Dialogs. You spend Most of the time in the project window itself, the mixer, transport, and all the plugins. None of those use standard controls. If they did, they would've pleased all blind users (assuming they do implement the accessibility features you mention), but turned off everyone else because of the bland, ugly and difficult to use UI, not to mention that there are only a very limited set of standard controls. I doubt Reaper would be the success it is now, if it were so.
Having said that, hey, this is Open Source! Like Reaper, I am willing to support accessibility if you, or anyone else is willing to do the extra effort.
[1]: https://osara.reaperaccessibility.com/