Because when you get older you make less of many hormones, including testosterone. You make more of others like estrogen. Testosterone and estrogen regulate body composition (among other more obvious things). While an athlete can train hard and eat right to lose weight, and will lose fat (compared to a sedentary person), how low they can go is determined in part by their hormones.
Because that's how it works. Women have more body fat (on average) than men. Presumeably that's because they need additional resources for producing children... women who didn't start out a pregancy with enough body fat to support a child didn't reproduce successfully and thus the genes for low female body fat did not get passed on. Or it may be because god likes curvy women.