It's a bit like when you buy a meal at McDonald's. If you buy a cheeseburger, fries and a Coke separately, it's going to cost more than if you buy all the three as a value fare. It's the same here. If you buy an ultrabasic and then add everything you could potentially add, it will cost more than if you just upsell to the next higher fare class, which is an economy fare.
That's what many do. If they want to bring this, then they buy the economy fare and they get it all as part of it.