Yeah, I really don't get how purchasing a car is worthy of congratulations. I even felt weird about it when people told me that about buying my house a few months ago. I had never bought a house before, so I wasn't expecting all of the "congratulations" we got, and I thought it was kinda strange.
Anyway, what exactly are you congratulating? I mean, like the above poster said, if this was a first car purchase where someone worked hard, saved and paid for it that's one thing. But just replacing a previously fine car with another one? That's not really an accomplishment in any way. It's just signing some paperwork.
I've also noticed that it's not worthy of congratulations when someone just upgrades their old Civic to a newer Corolla. So it seems to be tied to the nature of the car that's purchased. Certain cars are more worthy of congratulations than others. But even that isn't an accomplishment. Everybody knows that a BMW or a Benz is a "nicer" car, so it's not like it required some sort of expertise to make the decision.
My theory: congratulating someone on a car purchase is effectively congratulating them on their acquisition of a status upgrade. Even upgrading the BMW is saying, "My status has increased because I have leveled up my car. Not only do I have a BMW, I have a NEW BMW."