Dinosaur lifespan?

I would say anywere from 20-40 years for a large theropod. Small theropods probably had a wide variation in lifespan similar to modern birds.

Sauropods, at least the large ones, probably lived from 70-80 years old based on growth ring studies.

As for Ceratopsians there is not much info on lifespan for them at this point.

Why do theropods live so short?

The oldest T.rex was Sue at 28 years old. The biggest theropods therefore lived in the 25-35 year range, which isn't that short.

I just thought it would be like 50 or 60

but, i guess they lived fast and died young.