It’s just a quirk of the game.

If you look at a real property that you would rent out in real life, you subtract all the expenses from the rent money received. The remaining profit is called cash flow. Unless you buy the property outright with cash, the biggest monthly expense to the landlord is the mortgage payment.

After you pay the mortgage payment each month, the amount of equity that the property owner has, increases by the amount that the mortgage has been reduced. For example, if the property is worth $100,000 and the mortgage balance is $60,000, the owner has $40,000 of equity.

Ideally the property will also increase in value over time which also gives the owner more equity. Say the property owner still owes $60,000 on the mortgage, but there is a huge spike in housing demand and the property would sell for $120,000. That would mean the owner would have $60,000 in equity.

You can go to a bank and get a second mortgage to tap into this equity. Basically you want a loan for $20k, you take it against the equity in your home. Now you have 2 loans totaling $80,000 and the value of your home is $120k. Meaning you only have $40k in equity and $20k in cash that you borrowed. This would also mean that you have a higher total mortgage payment to pay the extra loan you just took out. So your cash flow on the property would be reduced.

Now let’s get to monopoly the game. In order to keep the game from getting too convoluted, there are not mortgages that need to be paid every month or turn. To simplify everything, they made it so you have to buy properties outright with cash.

The game also realizes that each property has an equity value that you can access by mortgaging the property. Since the game isn’t designed to have conventional mortgages, they can’t just increase your debt and reduce equity in exchange for cash like real life. So they just give you cash and reduce your cash flow like in real life. The way to do that in the game without getting too complicated is to just flip the card over and not allow you to collect the rent in the game. The net result is similar to the property owner as the money would just be paid to the lender and they wouldn’t get to keep as much.

The key difference is that the person landing on the mortgaged property gets a free pass. In real life, the person would always pay rent. It’s just a matter of how much the landlord is required to pay to service the debt.