It's a number that you get when giving your resources to someone in order to join Nakamoto scheme.
Thus doesn't the value of your resources get transcribed to bitcoin. Isn't this how gold gets its value? If (hypothetically) suddenly people said gold is not useful, then its price would drop because the value of the resource (our time + energy) that we are willing to pay for it is less.
Another reason could be the supply of gold increasing, if were really thinking long term and about the upcoming age of space exploration, gold might not be the best way to store value for humanity. There is just too much of it.