He didn't (invent the Windows GUI)... Xerox did!
Steve Jobs easily acquired it from Xerox, because those Suits couldn't recognize what they had.
Bill Gates copied the GUI concept to what Windows is today... and made all the bucks.
Either that, or he IS the Anti-Christ!
Sure sucks up all my time trying to get (and keep) these peripherals, OS's, networking, VPN, WOL, RA, RD, etc. working like Microsoft claims.
Bill Gates more or less took the idea from Steve Jobs(Apple); He's fairly good at marketing and so pushed Windows 1.0 towards the masses before Steve could get his version out(with the help of a nice big DOS contract with IBM).
Why did Steve invent the idea ? Well even that point is mute.......
Nobody knows why (The Inventor) will invent something! since the invention in question doesn't exist
But in hindsight (looking back on why something was invented) Steve probably wanted to make computing more user friendly/interactive. Bill saw that too and what potential it may hold, and WHOOF,BOOM -- exponential growth of Microsoft!!