"man years" is one of many statistics used to sound really impressive even if it's not portraying anything impressive, because the number scales so well. "60 man years" is conceptually a long time, but it can just as easily be one person working 60 years or 30 people working two years (which, from that PoV, isn't a tremendously large project).

My favorite joke about the issue:

Q: What's IBM's definition of a "man year?"

A: 730 employees trying to get a job done before lunch.