One of the functions of money in an economy is that it's serves as a store of value.
Yea, money is in no doubt a store of value but it is not a good store of value because it loses its value over the time. Not many people will want to store their value in money rather they would like to use money for exchanges of value rather than to store the value.
While money is an asset that can store value, and also Gold and silver for example, acts as store of value.
Gold and other valuable assets are good store of value especially gold because it is in appreciation value over the years. Bitcoin can be a good store of value because it is a hedge against inflation unlike fiat money.
Money can indeed be a store of value if it is only to be kept for a short time. but Money cannot be a store of value in the long term. Because inflation continues to undermine the value of the currency itself from year to year.
Money really can't retain its value in the long run. In contrast to Gold, whose actual value has never been affected by inflation. And gold continues to be able to maintain its value over time even for decades or more.
For Bitcoin I prefer to think of it as the best place to invest in this digital age. But for a store of value it is still very risky to do.
Apart from money losing some value due to inflation on the long run, money could totally turn to be a bad store of value if there is redesign of currency. You might have saved your money locally and have a long journey, after so many years you will discover that there is a change in currency denominations or design and this will render all the money in your possession useless.
I have to also agree with you that bitcoin is not a good store of value because of its volatility, but it can be used as a hedge to inflation.