PEGI is an industry trade association like the ESRB and MPAA (for movies). The MPAA realized a while ago that if you have a ratings system setup by the biggest companies in the industry, you accomplish two things:

Get government regulators off your back
Give independent creators a higher rating to keep them niche

In the MPAA's case, they do give indy films higher ratings, and no, it's not just because indy films have more risque content. That was accounted for by comparing to a popular independent rating system.

The ESRB was started when some members of Congress (Lieberman and Kohl, both Democrats) glanced over at video games and acted like they were ready to pass legislation. That legislation would likely never have survived a First Amendment challenge, but the industry would rather not go through that. Besides, it presents an opportunity to do the second bullet point while looking good to their core customers by doing the first. All the various CEOs call each other up and the ESRB is formed almost literally by the end of the day.

And now we have PEGI doing the same thing.