They aren't, necessarily. Eating too many simple carbs spikes your blood sugar & doesn't give you lasting satiety. I run, too, but I can tell if I've eaten like shit before a long run, it really makes a difference in my energy levels - like a bunch of white bread & cookies versus whole grain bread, peanut butter, & pears.  A lot of women my age (mid to late 30s) do low-carb & swear by it, but I don't want to smell like beef & sometimes they kind of do...