There is another aspect to this people might be missing:
If every phone that is shipped must come with a charger, but half the chargers are never used, then the marginal value gained by customers from the manufacturer improving that product is also halved.
So why bother making the charger good? Half the people won't use it! They'd have to pay for something that's not a direct benefit to the product they do want to buy: the phone.
This is one of the reasons the chargers that come with typical electronic devices are so bad. So very very bad.
They're big. They're odd shaped. They can't be plugged in side-by-side. They're heavy. The don't provide much power. They're inefficient. Etc... Some are downright dangerous.
It's a race to the bottom where noone wins.
Here's the thing: I like the "fancy" chargers Apple makes. They use very advanced technology such as gallium nitride circuitry to miniaturise their chargers. They're tiny, yet powerful! They don't get hot. They're light. They're safe.
But I only need one or two, but only when travelling. At home I need none because the IKEA power strips have USB charging ports built in!
I'd much rather concentrate my dollars into a couple of really good chargers instead of a bunch of mediocre chargers that's forced upon me on a one-to-one basis with the number of devices that I own.
Also: I haven't used a wired charger for my phone in about a year. Why include a wired charger if the customer has a choice of charging technologies? Should Apple include both a wireless and wired charger? Should they include a car-kit too?
No. This is the point of standards: it lets you mix and match components at will instead of getting a fixed combination.