Ask HN: Why is the pricing page the most common call to action?

I am just wondering why most standard web 2.0 websites have the main call to action button on the homepage send the visitor to the pricing page?

This is assuming a pretty typical navigation structure for the site that includes a tour page. Even though ultimately you want the visitor to hit the pricing page and sign up, I would have thought the conversion rate would be higher if they went via the tour page first.

Why do people not have the tour page as the main call to action, with the tour page then trying to funnel them to the pricing page?

A good example is 37signals and http://www.basecamphq.com

Is this just a case of everyone following without questioning, or is there some solid logic/evidence behind it?