Because people don't want to do business with a company. They want to do business with you, or they will, when I help you get to them.
You've been to design school or you learned on the job. Either way, you build beautiful websites. And now, your clients want more. They want you to help them drive their business.
It's unfair. It's not your job, but if you're gonna do it, you might as well do it right, and while you're at it, you'll learn how to explode your own business.