Why? To sell you something. If the traditional web incentives were to disappear, the internet will become just ads, selling things & work.

The web won't die. It will just become even more commercialized than it already is. You might recognize most of the things listed.

Selling things: Restaurants, hotels, local businesses; Ads: more of the same

Work: a) Projects people collaborate on and need to share information b) Writing articles to boost a resume

And maybe sharing what people do, they do it for free, see social media.