Why can't old show's like boy meets world, fresh prince of bel-air, or full house have a reunion show?

There should be reunion shows where all old shows come back. The exact same characters should play their role like they always used to. They can reunited and start their show all over again. Like the fresh prince of bel-air, which is my favorite show, will smith and all the other people should come back and start their roll again. that would be awesome.