You can make more money overall camping MVPs like Stormy Knight.
Valk armors have been steadily declining in prices recently, and are now around 45mill, some have even been underpriced for less.
Sure you can camp valk day and night (like some valk squads), but your chances of getting a worthy drop (like a manteu or armor or the more easily sold card) are extremely slim, and even if Valk does wind up dropping something, it's a race between you and the squads.
Also, if you wind up joining a pro squad, you'll usually be subject to taking 6-12 hour shifts and/or a sort of round robin system. So now imagine finally killing valk, and beating everyone to the drop.
Guess what? Its not yours, since it's someone else's turn to get the loot, and you still have 3 more people ahead of you to get drops.
Don't get me wrong though, it's a fair pooling system and you're guaranteed drops, but it's not really worth it if you're looking to make loads of zeny these days.
So really, these days Valking is just a decent side income and only something to engage in if you've already got a good main source of income, or already have a decent sized wallet.
Back in the day though, it was the way to make billions, since you had the warmth glitch to abuse and little competition.
However as I said before, that just isn't the case anymore.