Battery consumption and antenna efficiency are two major pain points. iPhones suck battery like a horse drinks water when in hotspot mode, and the large surface area of even a small MacBook Air would allow for some pretty interesting antenna design.
And it isn't really ironed out to behave in Germany where on a train you have frequent losses of phone connectivity. Every time it loses signal, the hotspot drops out and disconnects.
This is a weak argument. Qualcomm is charging for iphone and ipad too. They could do it if they wanted.
The real reason is Apple wants you to buy an ipad for on the road. Laptops, according to them, are strictly for office/home usage where wifi is available.
And it isn't really ironed out to behave in Germany where on a train you have frequent losses of phone connectivity. Every time it loses signal, the hotspot drops out and disconnects.