Tons of people have done it, and what the average non-technical user does is trivial on all of those operating systems. Anyway, Linux has half a dozen desktop UIs - referring to the user-friendliness of Linux as a unitary thing just makes it seem like you don't have a lot of experience in what you're criticizing.
edit: The best thing about Linux for non-technical people is that the GUIs don't change radically (for marketing purposes) every year or two. Once you get used to something, you can generally keep it, and changes and modernizations will accumulate slowly instead of forcing an adjustment all at once.
edit: The best thing about Linux for non-technical people is that the GUIs don't change radically (for marketing purposes) every year or two. Once you get used to something, you can generally keep it, and changes and modernizations will accumulate slowly instead of forcing an adjustment all at once.