If I had to guess, three reasons come to mind :
Firstly I'd say because they probably first design the product and worry about the current it requires later. Then they just design the needed charger to go with it.

Second reason could be that when they include a charger in the product they can make the whole package more expensive. So they probably gain a bigger profit that way because the charger doesn't cost as much to produce as the price premium they're asking for it.

Third reason could be that when someone looses their charger or it stops functioning, 99% of the people will buy a charger of that specific brand = more $$$ for that company.

I'm just guessing though.