Because its value goes up.

Which screws over any business that has a time lag between buying parts and selling the final product. If the value of money goes up as the product is manufactured, you have to reduce the price. If money deflates ~faster than your profit margin, profitable business is impossible.

[1] after expenses and other minutia

You can hedge to fix that problem, if you are so inclined.

Hedging might fix small amounts of temporary deflation, but it isn't going to fix the fundamental problem of not being able to recoup the initial investment in materials.

The only way to fix this would be to require payment before manufacturing the goods so there isn't a time lag.

A currency is a "medium of exchange," i.e. you can use it to buy and sell stuff. If your "currency" might buy 20% more stuff tomorrow (like Bitcoin, sometimes), you won't use it to buy that stuff today. Instead, you will hoard it and use something with a more stable value to buy stuff.

Maybe not "buying stuff" unless you truly need it would be a boon to the environment. Endless growth is for cancer, not the economy.

Agreed, but I kinda like buying food and shelter, and it's much easier to do so with dollars or euros than with barter goods or magic digital beans.

Yes, that is the standard econ. argument. I'm not completely certain that I buy it, though. Yes, people may wait to buy things. But ultimately people still want to consume stuff. If they time shift that consumption, who cares? What good is wealth un-utilized?

Let's say you own a pile of X. If the value of X only fluctuates a few percent per year, like a major currency, you will freely trade your X for stuff, because you can always get more or less the same amount of stuff in the future. But if the value of X is changing 10% or more per year (or per day), you will either exchange as little X as possible if it's going up, or dump it all if it's going down.

That's only true if you have the expectation that that volatility will settle in the future, though.

Well sure, tulip price rise too if you can convince people to buy them. It's a pretty undesirable property for a currency though, unless you never plan to sell anything to anyone who uses a different currency.

I guess what I'm getting at is that it is deflationary as a currency which is bad. As an investment vehicle or value store it seems to be unmoored from any fundamental value.

Deflation increases the value of currency... and debts that are denominated in that currency. Highly regressive systems that would accelerate the increasing wealth gap probably sounds appealing if you're on the privileged side of the gap.