It is to make the best product they can and avoid pitfalls of supporting millions of existing distros. If you had put in a lot of effort in making the hardware,

It is to make the best product they can and avoid pitfalls of supporting millions of existing distros. If you had put in a lot of effort in making the hardware, would you just say to your buyer: "just download and install any linux os" and deal with the endless user problems that will, as a rule, appear? No, you provide the best system you can so everything works as smoothly as possible so the users are happy and do not call support or (god forbid) return the goods.