[kevincox]

Because the URL is naturally short so you have to be very selective about what you put in it. So if the URL is "on topic" the page likely is. Just like if the domain is "on topic" it is very likely the site is, because it is short and hard to change.

IIUC some of the biggest factors that Google uses for the page itself (network effects obviously play a huge part) are domain, url then title. If you notice these are fairly space limited and user visible which means that it is harder for the website author to spam these with possibly relevant keywords.