Because if they sold it as a 20 gbyte unit, but priced it as a 64 gbyte unit, they'd get called out for that instead. Damned if you do; damned if you don't.

It's not the user's problem that the OS takes up that much space. The user is already paying for the license. He shouldn't have to pay twice (that 40 GB of SSD space is pretty expensive).

This is the second place in this thread you've said this, I don't understand what you could possibly mean.

Do you think Microsoft could somehow throw the space in "for free"? How could you even tell?

Then Microsoft needs to make an OS that isn't so bulky, tailored for tablets. Apple was able to do it. There's no reason Microsoft can't.

Compatibility is a huge reason why so many people still use windows. Compatibility with older applications and formats. That's what takes up so much space.

They promise a 64GB flash chip, and that's what they deliver. Unless they are concealing or deliberately lying about the user-available space, which doesn't seem to be the case, I really don't see the problem.