I'll echo a lot of the comments already made above.
Many years ago, when I was first getting into trading, I did a lot of testing using moving averages, including moving average crossover systems. I discovered a few interesting things along the way that propelled me to learn more about the markets, but I didn't find any statistically significant patterns when using moving average crossover signals on equities. I distinctly remember being tremendously (over-) excited one day when I thought I had found a great edge, but fortunately I had enough sense to go learn enough statistics to properly test that hypothesis before putting money on it. I saved myself from losing money and gained knowledge in a then-new-to-me field to boot.
Fast forward fifteen years and I now have three models with high statistically significant and tradeable edges. (Personally, I define having an "edge" as having a positive expected value of profits and losses; ie E(PL)>0, PL=all profits and losses due to this strategy. I go beyond basic statistics to verify that E(X) is truly statistically significantly positive, including various checks of robustness, using both parametric and non-parametric statistics, etc.) One of these models involves the 200 day SMA, but the moving average is used as a filter, not as a trading signal. This model could be viewed as the combination of two (related) models: when the price is above the 200 day SMA, I use model M1 and when it is below the 200 day SMA I use model M2. The reason for choosing the moving average is, in fact, rooted in the rationale for the formulation of M1 and M2 in the first place. It's not arbitrary.
I'll end here. I'm being called for dinner..... lol