Month over month volatility is a poor reason to put money there. Long term average growth is a good reason. Over the 10-20 year periods in the last century, what widely available asset classes perform better?
The America of the past 100 years has been quite exceptional. Using its history to predict the future is not a good idea, IMO.
The problem (or uncertainty) is that both the US and Germany left the gold-backed systems during those years and the exchange rate between our currencies changed quite a bit during the 1914-1924 period (which is probably why your parent chose that year).
If you notice your image's description, it is denominated in 3 different currencies, depending on the years you are looking at.
Im sure the result would be much better then.