I'm an engineer and a biker. If you only look at the numbers or try to run simplistic calculations, it's easy to think that bikers are just deluding themselves.
But there's more to it than just numbers on paper, and there's more to biking than just riding in a straight line from point A to point B. Carbon fiber frames aren't just about saving weight. The Carbon Fiber has different flexibility and damping properties that result in a different feel, for example.
My engineer friends make fun of me for spending $1K on a set of carbon fiber wheels, until I let them ride the bike. It only takes a few pedal strokes to feel how the reduced rotational inertia translates to faster acceleration. The advantages are even more significant when jumping or throwing the bike around, because the bike's moment of inertia is reduced significantly.
Carbon fiber handlebars don't save much weight, but the damping properties make for a minor reduction in fatigue. Carbon fiber Derailleur arms will again only save a few grams, but they're more likely to spring back from impacts than metal pieces that will simply bend. More expensive shifters have better bearings and cables that make for more precise shifts, meaning less fiddling over time and less mental effort to make sure I'm only changing 2 gears instead of 3.
The list goes on and on. Casual observers think we're just being silly and fooling ourselves with expensive parts. Meanwhile, I'm still buying the expensive stuff, enjoying it, and having zero regrets. At this point I just don't talk about it with any non-bikers because so many of them think they've got it all figured out.