Gravel road/adventure bikes are based on the endurance road bikes and typically have taller head tubes, a frame geometry that sets up one for upright riding and they have room for wider tires for stability and added comfort.

A lot of people buy them because they want to ride both on pavement and off road and they want to have fun.

Unless you're intending to compete in the TdF, getting a lightweight bike that puts you in an aero position isn't what most people want out of a road bike.

That's why the increased choices in the market these days is a very good thing, indeed.