Fluctuation is inversely proportional to the square root of the sample size, however.
In other words, with a sample 4 times as large, the result fluctuates only 1/2 as much from the true win probability.
Of course, if we have 5000 simulations for a deck with a 10% win rate, the standard deviation of the difference between the simulated win rate and the actual win rate is only 0.4%, which means about two thirds of the simulations will fall within 0.4% of what the actual win rate is supposed to be.