Why is it when you're alive tooth decay is prominant but when you die your teeth doesnt rot?

Because you aren't eating or brushing your teeth when you are dead. When you eat, things like sugar break down your teeth. When you bruch, you take away a protective layer of enamel (sometimes).

Why is it when you're alive tooth decay is prominant but when you die your teeth doesnt rot?
Quite simply because when you die, You don't eat. This is sort of like, When you drive your car, it will get dirty, but if you leave it in the garage, it stays clean. However, to a degree, teeth DO rot after death.
Reply:I believe that it has to do with the fact that you don't eat when you're dead. Lots of foods are corrosive, in addition, foods generally break down in your mouth and create plaque which allows bacteria to thrive in your mouth causing things like tooth decay...etc.
Reply:This is a guess but I think it's because the organisms that cause your teeth to decay depend on you to live. They need your saliva, your body heat, and the oxygen that you inhale in order to do their business. I don't really know though but that's the best thing I could come up with.
Reply:Bacteria also feeds off food you eat + flesh and blood (live cell) which proliferates bacteria growth. Teeth may still rot to a certain degree with the dead unless cremated.