That's because "handmade" was a proxy for "high quality", in situations where machines lack the skill to meet tolerances and fine details and customization that humans can.

I never got the appeal of that honestly, less for something like a BUGATTI. Not like an artisan hammered the panels into shape by hand, or somebody manually bored those cylinders.

In fact it's probably something completely incidental to the entire thing -- they're not assembled by hand because it's better, but because not enough of the cars are made to make automation make sense. It's just a pragmatic business decision, which then some marketer figured out how to spin.