When I mean constantly I mean 24/7. You have rest days or even if you lift every day, you will still be not lifting for 22+ hours of the day. Even the most avid lifter is only doing it 5-10% of their life. The rest of the time gives the body the time to recover.  A fat person is literally doing it 24/7. Their body never gets a break.  The other factor I didn't mention related to this is form. When you lift properly it is controlled, carefully designed movements to avoid injury. When you're fat and carrying all that weight around you don't have this option