Why do people contribute to Wikipedia?

Because everyone knows that everyone knows that Wikipedia is the canonical source of factual information on the web.

This is an effect that has gravity-well dynamics; it accelerates as it goes.

If you could rewind the clock, it's not a given that wikipedia.org would be the undisputed winner, but it is a given, in my opinion, that something like wikipedia would happen.

I wrote my undergraduate thesis on the emergent power dynamics of Wikipedia's user community.

One editor I interviewed was retired, and every day he'd go to the local library and transcribe a few articles on fish species from an out-of-copyright book.

The information was at the time nowhere else on the internet.

It was a point of pride to him that he was the one bringing that information online.

And where else would he post that information than on Wikipedia?

A similar dynamic: a few years ago I worked adjacent to User Street View on Google Maps.

Users could upload panoramas they took, and in limited circumstances they'd show up in Google Maps.

There was one example where a particularly motivated user bought a few GoPros and duct taped them to a broomstick and then drove around his small island nation to collect panoramas.

His nation was one that hadn't been prioritized by our official data collection process.

Someone in leadership commented "He must really love Google Maps!"

To which I replied: "No, he really loves his nation. He wants to put it on the map, and the map that everyone uses is Google Maps, so that's where he puts it."

Editors on Wikipedia are a very small number of highly engaged people who have devoted significant time and effort to Wikipedia, which confers them authority.

If the only way to climb the hill is to do the expensive/valuable thing, then being at the top of the hill does imply a legitimate authority.

The Wikipedia article's talk page is a great record of the most interesting / relevant / incisive discussions on the internet on that topic.

Nowhere else can you see, so perfectly distilled in one place, the constantly-evolving knife's edge of society's collective understanding of a given topic.