Most people want a car that they can actually buy. How long until I can walk into a Tesla dealership and have a Model 3 within the week?
And ordering (and waiting months for) a car is the norm in Europe. I actually think the buy-something-off-the-lot culture is why we get so few options on cars in the US; just a bunch of shades of dull with dull options bundles.
There's a bit of legal stuff behind it too; my understanding is that you CANNOT be legally held to buy a car that you haven't seen (which, of course, seems like an obviously good thing). But a side effect is that if you order your Porsche in pink with purple polka dots and no stereo, then decide you don't want it when it arrives, the dealer may end up with a car that is not so easy to sell.