Because as a living being you CAN NOT stop consuming (and spending).
You could delay some consumption, but you can not stop from consuming goods completely.

For example:
You have only BTCs and need to buy food to eat. You could delay consumption for a day, maybe three, but then you will be hungry and the desire to eat will probably be stronger than the desire to profit from a future increase in value of your BTC. Or maybe not and you will die starving. You will probably just use the minimum quantity of BTC you could spend to buy ramen and delay buying Champagne and caviar for the future.

This is an extreme situation, but there is a large number of people with different (often very different) time preferences, so there always be at least a small flow of Bitcoin to be exchange for goods and services or fiat.
If I acquire Bitcoin just to pay for my new tablet, because I can only use Bitcoin if I'm in Ghana and want to pay a shop in the US, my time horizon is very short. If I buy Bitcoins because I want protect my savings from inflation in Argentina, my time horizon is much longer.