Why is bitcoin always compared to fiat?

I get it, inflation is an issue. But fiat cash is not an investment. If bitcoin is considered an investment, should it not be compared to other investment options?

I constantly see it compared to fiat. If a person is deciding where to invest money, it's most often not in fiat. It's in stocks, bonds, real estate, etc. So what am I missing here? Is there any merit to these comparisons between bitcoin and fiat?

Note: I understand that bitcoin can be used as a currency, however, most people (especially in this sub) view it as a buy and hold investment.

Edit: most of the comments seem to think I'm asking why we value bitcoin using fiat. That is not the question. I understand every investment is valued using fiat (the dollar). What I'm saying is if bitcoin is viewed by so many as a buy and hold investment, it should be compared to other buy and hold investments. So instead of saying "fiat is trash so invest in bitcoin," the discussion should be "why should I invest in bitcoin rather than stocks and real estate."