The pool of valuable stuff and services grows over time, in general. There is a fixed amount of bitcoin to represent the value of all valuable things. Therefore in a world with only bitcoin, a bitcoin grows in value over time and is more like an investment than currency. This is like if interest rates were always stupidly high, because why lend out your precious automatically appreciating asset when it makes you money just by sitting there? If people struggle to get credit (by borrowing) it becomes super hard to buy cars, houses, educations, etc. Imagine paying a mortgage but with credit card rates. It would be very bad.