Because stock tires are made in Pakistan and are not of good quality. They tend to become hard after some time and generally last less than branded, imported tires.
People even buy used tires, as long as they are ' Walaitii ' and ' Japani ' imported.
The reason for almost every brand new car having those tires is : Pakistani automobile manufacturers/assemblers are bound to, because, Pakistan.
The only car that is in my mind that uses other tires is Suzuki Swift DX and DLX/DLX Auto.
Consumers usually want to go for bigger rims or different profiles as well, so giving the tires in exchange is a good deal.
Usually, consumers never go stingy on tires- They are the only thing that grip you to the road.