and i see fat people drinking it all the time so it doesn't seem to be working
That's because they're usually ordering it with a Double Big Mac combo ;)
I've always found it funny when people order like that. As if the diet pop is gonna counter the 2234872184732 calories of a double big mac you're about to wolf down. Not to mention the fries (which of course has been super sized!)
Have you ever actually looked up the numbers? In a Super Size (etc.) meal, the soda has more calories than any other single element, and they are all from sugar. Cutting it out might not make them svelte, but it's the single biggest improvement they can make in that "meal".
Anyway, the problem is that they're not just eating one meal. What got me to stop going to McDonald's was going in to get my one sausage biscuit with egg and cheese and standing in line behind a woman who was so fat she was having clearly audible breathing problems. She got the diet soda, too... and two McGriddles. Now I see her looming there, breathing louder and faster than Darth Vader, every time I think about eating at Mickey Deeznutz. Also there's the Pennywise/Ronald meme to keep the idea fresh. Mind you, I'm still pretty fat. I may be 6'7", but 275 is still fairly tubby. I only have so much room to talk... But then, I could have put on her sweatpants, she couldn't have got into my jeans.