There are lots of physiological parameters with set-points, such as body temperature. The problem for weight gain/loss is that instead of one set-point for body weight itself, you have maybe 5-10 set and operating points that are indirectly related to body weight, but not direct measurements of it. They don't all have to be "working right" to keep you healthy, but if too many become disordered at once, you're gonna have a problem.