EU institutions force them by controlling the policy. Sure, if you have a few millions to invest, you can go elsewhere, but if you have billions, you are most usually working with other people's or institutional money, and are fairly limited by various laws and policies, and large part of that money practically has to go in some sort of government bonds. Then, you can not just disregard those AAA ratings at return * 2 because you'll have trouble explaining that you do not trust the ratings...

Which is why people are so angry at the ratings agencies - they are private institutions that were supposed to be giving honest information, and they have misleading AAA ratings to investments that did not merit them.