If you want to make it less vague, you can read Keynes.
It's inequality that is the important one, money printing doesn't impact it (except for it impacting inequality). In simple language, people don't want to spend all their money on consumption (the "demand is infinite" you see on econ101 is an approximation), and so when only two dozen people have all the money there aren't many things you can sell and turn a profit. But those people still want to invest all the money they aren't using, there is just nothing to invest into.
At the turn of the 19th to the 20th century, explaining this was a huge open problem in economics.
I probably should generalize my thoughts though to say “expectation of economic growth” (instead of just “money printing”) seems to me necessary to yield “opaque market insanity”, as opposed to “transparent evil sanity”.
As a thought experiment, consider a (practically impossible) scenario where there is universally no expectation for long-term economic growth/contraction — regardless of whether it’s “real” or just monetary. Then by definition, a long term market simply cannot exist at all. No amount of wealth inequality can cause market insanity if there is no (long-term) market at all.
Wealth inequality in such a situation can still yield hoarding, domination, conquest, control, scams, manipulations, etc. But I wouldn’t call that “market insanity” so much as “evil sanity”.
In practice, the real impact of wealth inequality on the common people would likely be the same either way. However, without long term economic growth/inflation, the “sane evil” of the greedy wealth can no longer hide behind the veil of “market insanity”.
As far as I'm aware, in 2020 the reserve requirement in the US was set to 0%, and it has not been changed since then.
Those private banks can print that money out of thin air because government allows them to. And the government officials (many formerly financial executives) allow them to because they “have to” to prevent “disastrous” private banking/financial collapse.
But if you or I wanted to play the same games to print our own money they way they do? No, that would be wrong and dangerous and illegal!
So it’s pretty clear that both government and private financial institutions are tightly coupled partners in a mostly corrupt, intentionally obfuscated shell game that primarily serves to keep money and power steadily flowing into the hands of the already wealthy and powerful.
Just look at who is actually held accountable for financial crimes. Some individual trader that finds and exploits some glitch that allows them to profit from the wealthy? Straight to jail. High ranking institutional powers (government and private) that implement often illegal schemes that continuously siphon wealth from common people into their hands? Slap on the wrist at most.