What irritates me so much is that all of these things are bolted on afterthoughts. People point at their "beautiful" perfectly liquid models and then when you tell them the real world is "ugly" and illiquid they get angry and tell you, you are wrong.

I mean the most classic example is the theoretical barter economy that doesn't exist because you need money to lower transaction costs for a high degree of the division of labor (which isn't disputed). The reason for that is that economists came up with this theoretical model and the text books want to teach barter first and then money which gives the wrong impression that barter economies actually existed. No, people either managed the economy the way we manage it internally within families and corporations or they straight up used a commodity as their money or some hard to forge symbol like precious metals. There is not much in-between because barter is inconvenient as hell and it causes social difficulties because the fairness of trades has to be evaluated very carefully, opening up the potential for conflict.