Jews will pray at the site of a tomb, but not because it will "count" more, but because it will be more meaningful to the people praying. When prayer occurs at a site of a tomb of a sage, or rebbe, ect, some Jews believe that the sage, a master of the art of prayer will help the Jew pray to G-d in some way. Prayer at a tombstone, is to G-d, the same way prayer in your living room is to G-d. It is a matter of which area, or setting has more meaning to the person praying. Which is why a grave site is acceptable place to pray to G-d.
I have seen Muslims pray on sidewalks beforem are you saying that in a cemetary is not a place where you can pray to G-d? G-d hears your prayer regardless of where you are according to Judaism, it is a matter of is a cemetary a place where prayer will have more meaning to you. A place where a wise teacher was that I knew is a place I may pray sometimes. I never pray to him. Only to G-d. But I remember the things he told me about prayer, and about Torah when I am in the cemetary. Prayer becomes much more emotional when it occurs at certain places for me, instead of a daily routine it might have if I am in my room.