the value of something depends on the persons involved, time, supply, and demand
there is no anchor “perfectly stable” valued item you can use either, and even if it existed, it would not have the same purchasing power at any given point
for example, with bitcoin, which is obviously not a stable store of value due to it being speculated on, even if it was stable, it is not immune to for example trust in it being shaken up due to the difficulty of forks and quantum computing risks