As an (aspiring) architect I can tell you that it's done (I design houses in America at the moment, and a lot of them have a private WC) it's done to maximize efficiency just as the others suggested i.e. you can be doing your thing without occupying the entire bathroom.
But that doesn't explain why you can't have a washbasin in the WC. Otherwise, you still have to go into the bathroom aferwards to wash your hands. Assuming that, after you use the loo, you do wash your hands. (I'd hate to look at that door handle with a microscope, BTW).
I know Americans who have a separate room with just a loo and a washbasin, but there is always a washbasin. (My MIL in England has this, as well.)
I always feel a bit uncomfortable when I'm in the homes of people who I don't know very well, who have a "no sink in the toilet" arrangement. Especially if they offer me something to eat or drink.