According to Hermione's research, there are only 7 known Animagi from the last CENTURY. In fact, the ability is SO RARE that those who are able to successfully become an Animagus are required to register with the ministry. 

So..My question is: WHY are there so few? How is it that this powerful ability is so underutilized in wizarding society? And what happens to those who have tried and FAILED?

But perhaps most importantly: Why haven't the most powerful and gifted wizards like Voldemort and Dumbledore attempted it? Or..Have they?