Generally speaking it is taught in quality web design courses. However, thinking about it during the design process is generally ignored due to the attitudes of developers and corporate bodies, things such as 'can't be bothered because of time and cost' also 'it affects the design' etc.
There's an arrogance to these sorts of excuses that isn't a good look for companies. Sadly variations of this theme are seen across the spectrum of disabilities and accessibility.
Like it or not, all international, national and legally enforceable standards are in place for a reason, not there to be ignored!