Because rain, snow, heat, lack of fitness, security, amongst other reasons.

Cycling is my most common mode of transportation these days. I love it. But I also know it's not going to be practical primary means of transport for a huge slice of the population. And the "danger from cars" ranks pretty low in my list of reasons.

A bike cannot compete in terms of energy efficiency with a slow robot that delivers multiple packages to people along a route.

Human food is the most expensive fuel on the planet when you factor in agriculture.