Why does everyone hate the London?

The difference between the London and the Stonewall, of course, is that in the London white can play his setup against (almost) anything and still come out of the opening with an even position where he can try to outplay black.

Whereas the Stonewall system is nowhere near as universal. In the Stonewall, there are certain setups from black where if white continues playing the Stonewall he is just considerably worse, if not positionally losing right out of the opening.

As stated it's not ambitious, there doesn't appear to be any great point to the manouvre. It's just a way for white to play mindlessly for a while without throwing away the game. There are also no real classic games or situations to follow in the footsteps of and study.  

"but Carlsen..."

Professional chessplayers are under tremendous pressure to come up with something in the opening their opponent hasn't seen or quite understood completely before. If they end up playing the London it's more of a surprise weapon they have locked away ready to spring, most likely with computer-prepared novelties to hand. So it's suboptimal for them in theory, but might work as a surprise. That's a totally different scenario to Joe Average just playing it every time.    

The London doesn't prevent black from being creative.

There are many ways for black to play against the London. You can choose a different way every time.

Black can play a Slav structure. Or a King's Indian. Or a double fianchetto. Or a Queen's Indian. Or a Dutch. Or a Benoni structure. Or a Semi-Slav structure. Or a Tarrasch structure. Or a gambit ... Or ...

That can be said of virtually any opening.  That is, if you play poorly, and don't adapt to what your opponent is doing, you'll probably lose.

The Stonewall Attack...

https://www.chess.com/blog/RussBell/stonewall-attack

If black plays ...d6 and ...g6 then white cannot play f4. It is a positional disaster of a move. That isn't adapting, it's dodging s Stonewall entirely and playing something else.

This isn't the case with other system openings like the London, KIA or Hippo. There are certain lines from black which make these systems less favourable, but white can always come out of the opening with a reasonable position. Not the case with the Stonewall.

I am not bashing the opening, by the way, I've likely played it about 5,000 times as white, but I just don't think it can be used as a universal system in the same way the London can. 1.d4 d5 2.e3 is probably the only acceptable way of reaching a Stonewall, there are even problems with 1.f4 first since black can get in an early ...d5 and ...Bf5/g4

London is a great starting opening for players learning. It gives them confidence. I always reccomend it to newer players who join our club, and Nelson Lopez video 35 principles. It seems to help and thats what its all about is it not?

This is the video I usually get them watch when they are at the London stage. (in my very limited experience). I highly rate this fellow myself.

https://www.youtube.com/watch?v=nXyJdetptXg

It's funny, every single time I play against London players, I recieve a worse position out of the opening.

And yet, curiously enough, I normally win those games.

When I have the advantage in the opening, I lose. When I'm at a disadvantage in the opening, I win. 

This is a typical 1.d4 game albeit against low-rated opposition. But a lot of my games are a bit like this if the opponent allows it. 5/5, just played. I could give hundreds of similar examples.

Very similar one, just played. Not a London but relevant to comments here.

Here's me using the London to outplay a FIDE Master, in blitz:

I don't play the London all the time - I mainly use it against certain black setups. But when I do play it, I find it both solid and flexible.

The "boring, drawish" reputation is undeserved, in my opinion. A boring, drawish game depends entirely on both players.

Many people who have no imagination think that in certain openings, they're condemned to a dull game.

The London is boring to face. Queen's Gambit gives you a lot of possibilities while the London is the same 6 or 7 moves every time. Also because it isn't a straight up refutation like the Englund or Latvian Gambit meaning you have to grind your teeth to break the London apart. 

Also, I hate the Botez sisters because they are boring to watch and every time I even give them a chance, there is almost always a London being played not to mention how Eric Rosen, Aman Hambleton, Ginger GM, and Gotham all promote it. 

I played the London until I got to 2000, where I got stuck and changed my repertoire. I don’t think it’s bad, I mean it got me to 2000 soooo...