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Why is mean reversion difficult to trade?

High turnover, high costs. You flip your position too often

If the market flips against me, I just double my bet on the next play.

the market can stay irrational longer than you can stay solvent

Martingale is a sure-fire way to lose all your money rather quickly.

That's a great way to go broke very quickly.

Only if you don't have infinite money on the side, in which case it's guaranteed to be profitable

You also need the other side of your trades to have infinite money. But in any case, why are you trading at all if you already have infinite money?

Martingala with transaction costs are not profitable.