Fat people don't typically ride bikes -or not very far or long. Today most people don't ride bikes or go outside or do much except sit in front of a TV, computer, or gaming console when they aren't at work or school (where they sit at a desk and/or computer.) Most people's jobs are sedentary or at least not very active or grueling. We have undocumented laborers to do the hard work these days.

We have lower gears today because we have MORE gears so the bike makers can spread them out a little bit further and can afford to push the bottom down without sacrificing the upper gears or creating too wide of a ratio between them.