It's a cliche but as they say 'there's nothing harder than winning a won game'.
I think that's a foolish saying, it's not rational and just contradicts itself. If the game is really won then it should be clear what to do. If not, make cautious moves, WATCH OUT FOR THEIR MOVES, and eventually it should become clear what to do and then you can say the game is won. Don't try to "force" your way of winning "by rights".
Some games that appear won may be far from it, may actually be equalish, even if a person is up the exchange.
And for goodness sake even in blitz at least go one ply deep when analyzing your opponent's moves.
You bring up an interesting point ( reframe from the name-calling). If i am your opponent and find myself on the wrong side of your won game ! The tactical minefield you must pass through on your "path" to victory combined with the desparate all-out counter-attack that will surely be coming your way is because i REFUSE to believe my opponent has mastered the art of winning the won game !! 
The "foolish" saying was Tartakower that first say it and Keres repeats it at his book (Practical Chess Endings).I don't think they are "fools".
Have you any idea how many won positions have been lost even from grandmasters? Even from top class players? Even from world champions? If someone attempted to write a book about it , a tome would not be enough.
Winning a won position needs experience , technique and emotional discipline(for OTB games mostly). The last one is the most important and the reason many won positions are never won.
A very interesting 11 pages!!! research published by the Soviet(back then) Shahmaty Bulletin at November 1983, was dealing with that problem and the reasons.Studies have been made from great Soviets like Boleslavsky , Suetin and Botvinik.I don't think all these are fools.
Hey idiot. Those games weren't really won games if the GMs lost them. In the future maybe White will be considered or even proven to have won from the very first move, that doesn't mean it's practically won. Only by jumping ahead of yourself do you call a game "won" when it's not.
I never called those people fools, I said that purely in my opinion it's a foolish saying. This is just my opinion on using that saying. It contradicts itself, so I don't like it. They are obviously trying to use an idiomatic turn of expression and it is just not to my taste. I just said I did not like it.
Please modify your language accordingly. There's no need for language like that. Go to Michael Adams blog, the inspiration from the blog comes from that. He's rated over 2700 by the way, so if he is also being naive and you can demonstrate it, I'm sure he would be very interested to hear.