Average websites goal is now to keep you on them as long as possible. According to some metric folks, the longer you stay on a website the more money you spend there. Linking to another website destroys that metric.

Also if you are going to make a purchase somewhere, any website would try to get a cut of the money you spend by actually sending referral links to the product. So small websites that do not allow this service will not get linked so much.

On a metalevel it is thus that links or connections between items are information. Information is money. And as soon as that became evident links and connections also became more scarce.