Is there any real-world reason why someone absolutely has to spend a night at a $2000 hotel when they have five dollars in their bank account, or is it just "the rules"?
Monopoly was originally designed as a game to show the effects of capitalism. But the people liked the collection of money and the suffering of their fellow players too much so the original purpose was soon lost.
The game is a model of a subset of reality. So yes it needs rules. They may seem unfair, but real life can be a bit less optimal. And at least with this game, you can flip the board, curse at your (obviously cheating) little brother and storm out of the room. Real life has some limitations on that.
An obvious real-world reason is that hotels charge you for your stay whether you have the $2K or not.
The actual site of breaking the "real world" connection is that a (reasonable) person with $5 in the bank would stay home, yet the person locked in the body of a top hat or scottie dog is not permitted that option on the game board.
These sorts of concessions are part and parcel of board games. The simplification of the simulation is needed for the game to be both rigorously defined and still playable.