Accessibility as Standard
Most if not all professional industries have standards, rules, regulations and guidelines that must be followed. You may be mistaken but this does also apply to the world wide web. Why does it feel like the web industry is able to get away with not following their standards, Is it because they are referred to as guidelines? Does that somehow imply they do not have to be adhered to?
Why is it that it's not common practice that websites are not always developed to complete standards — complete standards includes accessibility! Accessibility is generally the most forgotten aspect - Don't take my word for it, just look at the WebAIM Million - and as you can see it is not getting any better.
Some not so recent updates in the law that makes accessibility a requirement (starting with public sector websites) is a huge step forward, but why did it take so long? Why does law need to be the driving force, and not meeting all your users needs. As a professional working in within the web industry you should be aware of the W3C and their standards/guidelines. It is a fundamental piece of the puzzle that is web development.
If you look at another industry. For example the trades industry. They each have rules and regulations for each trade.
Imagine having an electrician doing some work in your house and using the incorrect wiring for a light fixture. How would you ever know? You wouldn't until it was too late, maybe the circuit would short and cause issues, or worst case it causes a fire.
Electrician's have to be approved and work to regulations set out by law. You can even verify an electrician by looking at one of the regulators in the industry.
That's just a small example of another trade/industry, the effects of them not doing their work to a given standard can have devastating effects. The same drastic effects are not going to affect your website if it's not accessible to all users — but not having all users able to use your website is a poor acceptance.
If you run an online store you could be missing orders without even knowing. With so much choice on the internet to purchase things you are unlikely going to hear about the poor experience or lack of experience a keyboard user or a user of assistive technology had trying to buy from your store. It is much quicker for them to search for the product on another store and make their purchase via a more accessible store.
That is highly likely that you just lost a user who could be making regular purchases. Instead they remember the site they had an easier time and experience using and will continue to return.
The same can be said for all websites, if you are providing content - such as a news site if the site isn't easily usable with assistive technology users are using they will find the content elsewhere.