That's true, but the stock they receive is more in the notion of an honorarium than a payment for services rendered, and there's often mutual benefit aside from the stock (being on advisory boards is, or used to be, a high-status thing). Advisors don't so much try to value the stock they receive. But a lawyer or a web designer has to do that.

Also, the value a company gets from an advisor is long-term. Not the advice, but the NASCAR-sticker-like endorsement the company gets from the name on their website.