I agree with you. There is a certain "tough it outness" missing from some of the top runners.
It's almost like they're porcelain dolls- I'd like to see them race more frequently- Cross, roads, track, miles, 1/2 marathons. It seems there used to be more 30K and 20 mile races- perfect prep for a marathon.
Look at the racing calendars of guys like Durden, Rogers, Dixon, Fleming...