The reason is that a light and stiff bike is much more fun to ride! An expensive road bike is optimizing for fun and performance. A heavy old steel bike is optimizing for cost and reliability. If you are riding for fitness then fun is important.
My counterpoint is that I have a lot more fun when riding my relaxed steel frame bike at a moderate speed, than I ever had when riding a road bike fast. That means I ride farther and for longer, but it's the kind of fun that cannot be quantified in hard numbers.
I find the performance-chasing stressful, whether on a bike, in a car or in front of a PC, and we could all do with a bit less stress in our lives.
But the point is, an electrically assisted bike is even more fun. Or if you don't like any extra assistance, tune the power to just compensate for the weight of a steel frame so it feels like a carbon-fiber bike but at lower cost. Why isn't that the most popular type of high-end recreation bike?
You can tune so the acceleration feels like a lighter bike, but for braking and cornering, you're still going to feel the extra weight, there's no way to artificially lighten an object, you can't cheat mass and gravity, it's the same reason why a Tesla Roadster is a very different car from a Mazda MX-5 and the Mazda would be a fundamentally changed car if you made it electric.
Still, for the vast majority of everyday cyclists, the "purity" of the experience doesn't matter, they want to get from A to B in a practical manner. They want to see things and do things, the bike ride is not a goal in itself.
For the sports cyclist with aspirations based on their professional idols, they want to ride what the professionals ride, not a simulation. Personally I think the whole concept of trying to ape the professionals is a bit silly, which is why I don't do it anymore :-)
