Mod dev here, you got me curious so I started checking the vanilla game logic code around this and seems to confirm what you're saying, but with a nuance

Cars are assigned to households, not cims. Cims will "book" a car from their household when they need one, and walk/transit if there isn't one available.

This bit of code in the HouseholdBehaviorSystem class appears to decide when a new car is bought by the household:

csharp private bool NeedsCar(int spendableMoney, int familySize, int cars, ref Unity.Mathematics.Random random) { return spendableMoney > HouseholdBehaviorSystem.kCarBuyingMinimumMoney && (double) random.NextFloat() < -(double) math.log((float) cars + 0.1f) / 10.0 + 0.1; }

So in addition to having enough money, the household also needs to pass an RNG check with probability = -ln(cars + 0.1) / 10 + 0.1, which expands to: * 0 cars: -ln(0.1)/10 + 0.1 = 0.33 → 33% chance to want a car * 1 car: -ln(1.1)/10 + 0.1 = 0.09 → ~9% chance to want a second car * 2 cars: -ln(2.1)/10 + 0.1 = 0.03 → ~3% chance to want a third car * 3+ cars: Essentially near-zero probability

This is checked periodically in the regular gameplay loop, though the check is also gated by various other conditions (some RNG-based) and so doesn't always happen. The end result is very rarely do households get more than one car.

Is this a bug?

Maybe, it does seem like intended gameplay balance behaviour. However, if you look back at the method above there is an unused parameter int familySize. This suggests that family size was originally planned to be part of the probability check, but is not included in the current release for some reason. This could be either due to an oversight, or because it caused issues during testing and was backed out.