Why do so many car makers have subscription fees on the cars they sell?

Depends on what those subscriptions get you. Paying a subscription to use your seat heaters? Nothing but anti-consumer capitalistic greed.

Having a subscription to stream music or live traffic updates to the car makes more sense. This means the car has to have a cell data connection and SIM card, which is an ongoing cost. It’s unreasonable to expect the manufacturer to eat the cost of a cell data plan for every car in perpetuity, so it makes sense to pass that cost on to the consumer.