Markets are not and never have been perfectly efficient. This is why traders are able to make money - they use information at their disposal that indicates the market value is incorrect to trade against that value. By definition if the market value was always correct that wouldn't be possible. So there is a feedback mechanism from information to market values, but there are many reasons those mechanisms might not work. There might be other more attractive targets for money that would otherwise be used to take advantage of that information. There might be a lack of liquidity preventing funds being deployed at all. There might be short term risks deterring long term bets on accepted trends.

Imagine if half the currency traders in the world stopped trading. All of a sudden all of those inputs into currency prices would no longer exist. Would currency prices continue being traded in exactly the same way and weeks or months later be at exactly the same value they would have been before? No, because losing all those inputs into the market would reduce the information flow that drives market value. To look at the opposite case, does the world have the ultimate possible expression of the ideal set of perfect currency traders using perfect information? Probably not, so the market value probably isn't what it would be if there were, i.e. the current market prices probably isn't at it's perfect ideal value.