As
@Peteski said it used to be like that. As best as I can understand it the route Tesla have gone down, in order to ramp up production, is to reduce the available options and just manufacture a bunch of vehicles, irrespective of orders (I'm sure they have spotted that nearly 50% of orders are Blue, and less than 10% white (well ... before White was free ...).
So that means they will not allocate your car until pretty much at the point of handover. Personally I would have thought they could allocate it to you as soon as it came off the production line, but I suppose for LHD, at least, they may be delaying the allocation process so that they can decide if Norway handover is a bit busy they send all new LHD cars in current batch to Belgium instead. I suppose that is also true for RHD between Heathrow and Edinburgh handovers ...
Upshot is that when there is a backlog, like UK RHD launch, it is complete chaos. Once over that hump then build-rate is close to handover-rate and chances are that people will get new cars within a couple of weeks instead of build-to-order along with ship-across-the-pond which would (and used to) take several months.
Big Auto have been building cars for more than a century and have it all figured out. I expect Elon thought that hiring the most experience and sharpest minds on the planet would solve that but it didn't turn out that way. Handling the rate of growth Tesla have must be a nightmare - personally I would have put far more effort into COMMs and Back Office systems early on ... but it isn't my Gig.
The PowerWall Installation (I'm sure there are quite a few sold, but nothing like as many as Model-3 orders) is a really slick online experience for providing details of where you want it, what the available space dimensions are, and uploading photographs of your location, the type of power breakers you have and loads more ... its a huge form, and very slick. So they have that ... but for Model-3 you cannot even pick/change a Handover Appointment Time without calling and being on hold for 40 minutes ...