The biggest one are;

1. Spending more as you earn more(lifestyle inflation). People can’t live like they are student forever. Yet countless people do just increase how much they spend as they earn more.  This is one of the biggest reasons why even some high-income people are broke, studies have shown as many as 20% of high-income people are broke(living from paycheck to paycheck),and even more in upper-middle and middle class.
     
        This is a major reason for that, and
         there are two reasons why people 
         tend to this;
* What bad habits have eventually formed.
* Some are trying to “act rich” and show the world they have made it.

To be frank, i think instagram probably wouldn’t exist if there weren’t people looking to show off.

2. Getting into credit card debt,but not all debt is bad in some cases.Almost every successful business has used it,yet credit card of consumer debt is very dangerous.

3. Spending loads of money on depreciating assets like cars, and not assets that appreciate.

4. Saving rather than investing,this is particularly in some risky countries where the exchange rate and inflation risks are huge.Plenty of people have been wiped out by this in the past.

Ultimately,wealth and income are not the same,cash flow is also kind,as a potential liquidity for a portion of your assets is an emergency and its serve as an emergency funds.

Therefore,if somebody is worth $5M on paper,but it is all tied up in illiquid assets,that isn’t the same as having $5M in a liquid ETF portfolio.
Likewise, if somebody is making $500,000 a year but is spending $500,000 each year,that isn’t the same thing as earning $70,000 and spending $50,000.
In fact, it will make that person poor in many ways,even though general society would think they’re rich as they are earning 500k.

Yet in reality, they are just living pay cheque to pay cheque, and any unexpected events like ill health could put them on the breadline.