Several point I see here. Cars are now disposable utility objects with a planned life of 8 - 10 years [ask a dealer if you doubt this]. The industry has engineered them on that basis, everything is designed to last a fixed time with a specific "maintenance routine" then more or less all die at once. Bikes will last as long as you can be bothered keeping them. The derailer is a crude and terrible way to change gears in an engineering sense but it works and it's lightweight, a trade off between convenience and longevity. There were bikes [may still be] with shaft drive and enclosed hubgears but they were very heavy and not cheap to make.
The belt is a problem, although it can be light it doesn't like the wet remember very early m/cycles used, [and discarded] the belt drive. Modern toothed belts may solve the problems but they will require internal gears, again an expense for, in 99% of cases, very little gain.