This is an interesting topic.

I guess an investment is something in which you have some expectation of growth in value over time, while also knowing that there are risks of loss and volatility. You're willing to not exchange this thing today as it might get you a better rate of exchange tomorrow. You might be more inclined to pay for things with something fairly low in volatility yet slowly declining in value over time (fiat government money), rather than paying with the thing you think will probably buy you more in the future.

If we look at Bitcoin's chart, we see that it has done better in purchasing power over the last decade than fiat currency. So it validates the decision of many to Keep Bitcoin as an investment and Pay with fiat.

Due to crypto volatility and low adoption as a payment vehicle, it's hard for a business to accept it as a payment. They likely can't pay many of their suppliers or workers with the crypto they take in, and they're nervous about the value changing against them in the time it takes to turn their crypto income into fiat. This could change if crypto becomes more adopted, but it would likely only be more adopted if it becomes more of a stable coin, thereby losing it's investment allure (unless it is paying 20% interest, as one is).

One way in which crypto is used as payment is in buying other crypto! We swap from one we're bored with to the shiny new crypto which we think has more potential for gain. Or we buy a crypto asset in a game we enjoy. Or we buy a crypto collectible for its art value, or meme cultural significance. So when we stay within the crypto world, we do use crypto as a means of payment.

Just my random thoughts which may or may not be valid, inspired by your post. Thanks for sharing this topic and your thoughts with us!