In a nutshell, because they ordered 5-8 weeks ago.
Tesla delivers US cars at the end of the quarter. Almost always. If you ordered in mid October, you’ll likely wait until December. If you order in December, you’ll usually get a car in December.
Tesla abandoned “custom orders” ages ago, despite the nomenclature on their site. Cars are made in batches, and they do their best to anticipate demand. Since they live and die by quarterly delivery numbers, there’s usually some incentive to get people to “settle” for a car that might not be exactly what they would have ordered, but is available here and now and if they can seal the deal will count toward their quarterly delivery numbers.
I’m not sure where you get this “offer something they can’t deliver” sorcery. That’s obviously not the intent. Tesla WANTS to deliver you a car before the end of the year, because it benefits them, so they’re offering an incentive to take an inventory car that exists now. Seems pretty simple.