Because just about anybody can say they are a web developer. As an SEO I get handed all kinds of sites. You would be shocked what some companies put out. I'm not talking fortune 1000 but medium sized businesses that make good money and have over 100 employees. A company that size will have somebodies wife who fancies herself a web designer or the bosses next door neighbors kid who is in the 11th grade design the site. There are not that many web developers out there that really know their craft. They also don't take the time to understand the finer points of web design. Most of them have no concept of writing code. They don't know what the codes even mean. They spend most their time in a WYSIWYG editor and copy and paste snippets from code they found on the web or in a book. I mention SEO because SEO and accessibility have pretty much the same goals. If you do one well the other will have been taken care of. I am an SEO as well as a legally blind person.