Because actual money has sources and sinks. If you look at MMT for example, they talk about government spending not as being a way for the government to distribute money, but actually as a way the government creates money. When the government taxes money, it doesn't just collect it, it destroys it.

So, based on how much a government is spending and taxing, it's adjusting the supply of money in the economy. The fact that government spending is roughly the same year-to-year, and that taxes are roughly the same year-to-year gives a stability and flow to actual money. There's a constant demand for it because every year the US government requires that people and businesses submit $4.5 trillion in taxes. There's a constant supply because the US government spends more than $6 trillion.

Bitcoin doesn't have those sources and sinks. There's no constant demand for bitcoin every year to pay taxes. There's no constant supply as a government spends bitcoin into the economy. The "realness" of money is intimately tied to taxes and government spending.