It's because PEGI (and the BBFC before it) are backed by law, while ERSB (America's rating system) is not. This means that legally shops can sell games such GTA and COD to minors, however most shops refuse to do this as it could attract negative publicity. This doesn't change the fact though that when FOX says little Billy is in danger from GTA V they are technically, TECHNICALLY, right because Billy can legally go to the shops and purchase GTA V. This allows American media to basically pin the fault on game developers when children get their hands on violent games.
This doesn't really work in the UK. If someone tries to blame the game developers when little Timmy gets his hands on GTA V, all the game developers have to do is point at the PEGI rating on the box and the blame immediately shifts to whoever provided that game to the minor as they have actually committed a crime. When minors get drunk we don't blame the the people that made the alcohol we blame the people that gave them the alcohol. Essentially the anti-games argument has no teeth in the UK.
This doesn't really work in the UK. If someone tries to blame the game developers when little Timmy gets his hands on GTA V, all the game developers have to do is point at the PEGI rating on the box and the blame immediately shifts to whoever provided that game to the minor as they have actually committed a crime. When minors get drunk we don't blame the the people that made the alcohol we blame the people that gave them the alcohol. Essentially the anti-games argument has no teeth in the UK.