I doubt there are "lots" of buildings over 100 years old remaining in the Melbourne CBD. Those that do remain gain significance merely because they have survived.
It's less the CBD, and more in the surrounding suburbs. There are old cottages scattered around that weren't good houses when they were built, and aren't any better today. The land they're on is worth a fortune, but they have heritage status so they can't be redeveloped, yet they're also not worth renovating because they're so far gone.