I believe you don't see alternative materials because there is no real good reason to use something that's not wood; A wood bench properly constructed will do what it needs to. Seems to me like straying from that common path is going to result in extra time, effort or costs, for minimal gain. Unless you have some unique constraints or an exceptional scenario (maybe you got some free steel or concrete laying around?), I would stick with wood.
I did try to find LVL/Glulam for the bench top at the time when I built my bench but it was not commonly available for non-industrial uses.