It's like booking a hotel: Basic price will get you a room for the night, with all the common amenities, but if you want late checkout, you'll pay extra. Sure, they could fold that into the basic price and make it the norm, but if you know you'll leave early anyway, you'll be paying for something you don't want.

The metaphor breaks apart if you look too closely - for hotels, early checkout is a convenience since they can get the room ready sooner for the next guest, so they'll incentivise that, while the devs have already put in the work. On the other hand, the late checkout is a service of convenience while a DLC is an excitement feature, where the content is instead an incentive to pay more.

Either way, I feel like add-ons for games aren't too different from add-ons in many other industries: "This is the basic , with the price we feel we can charge for it. This here is an extra you can have for an extra charge."