The categories were defined by a user group many years ago when Zwift racing wasn’t a part of the program. The whole idea of using FTP w/kg as the performance metric, having only 4 categories and the w/kg and weight limits for those categories were made by that user group. Zwift has simply adopted those limits and category definitions with the only major update being their new method of calculating critical power for category enforcement.
Quite frankly, the whole system needs a complete rethink (ideally throwing away everything and starting again from scratch) but many people like to cling onto the old ways.
Maybe you’re right. Though reasons for riders in D can be for many reasons. Age, infirmities, laziness, whatever. As mentioned elsewhere, the reasons for low D participation in races is also because it covers a huge range of 0-2.5 wkg. Typically you’ll see that almost nobody under 2wkg even enters a D race – and not because they don’t exist. Take the outdoor bike club example. A 150 watt rider will travel at about 18mph. That’s not an unusually low of a speed to maintain but is a D cat in Zwift. Local bike clubs around here go down to posted rides of 13-14mph (and up to 22+mph).
I’ve always been curious if there’s any available distribution curve (percentages) of how many enrolled Zwifters there are, by 10 watts increments for average power. Might be interesting.