Very simply "the experience" is much better on mobile = we track everything you do, your location, we mine your friends info...And sell that for more $$$$. And either you don't it / or you don't care or it is simply too complicated unless you are in Europe (in that part of the world it is more risky to sell your personal info without your explicit consent).

Can't really do that if you use your laptop. At least it is harder and harder to get to it (trend to use Safari, Firefox, etc).