General Football Question: Can someone explain prevent defense to me?

I've been watching football for 60 years and I never have really understood the prevent defense. OK, I understand you don't want to get beat on the big play, got it! But so so so often, a defense that has played a great game lets the other team march down the field and either threaten or win the game courtesy of the prevent defense. A good quarterback can move a lot of yards in a minute or two, and giving them 8-10 yards every play is just asking to lose the game. I've seen so many games lost (no stats) in the final two minutes with the trailing team marching down the field and kicking a game winning field goal or making a touchdown against the prevent defense. Why not just stick to the defensive strategy that got you to two minutes with the lead? It seems to me to be a lack of confidence in your defense, or just don't want to have to go on TV and explain why you didn't have three guys standing on the goal line.