They don't if it's really an investment, which is basically defined as loaning somebody who does something productive money for interest. Out of it comes something of value, a product, a service, knowledge or information.
That doesn't happen with Bitcoin because the mtoney you used to buy Bitcoins goes to another person who doesn't use that money to earn something for you. That's why it's called speculating and that's why this board is called as such. We are speculators not investors.
You are basically correct. Investing is buying capital goods and labour with the purpose of producing someting that is useful to others (of course speculating that it is useful to others when it is offered some time in the future).
Buying bitcoin is speculating that others will demand it for a higher price in the market in the future. It is speculating also if you do not expect a higher price in the future, even when you expect a lower price. (When you demand dollars for the work you offer, you speculate that someone in the future will demand those dollars for a price not less than 2 percent lower than the current price... that is the dollar price inflation expectation). This is always so with money: any demand you have for money, is speculation that others will demand it in the future.
Speculation is acting on your foresight, based upon all your wisdom.