Man Utd are ranked as the richest as they have the most assets, not based solely on their bank balance.
Take into account the following:
Stadium and other club buildings including Academy and training grounds.
Shirt sales
Player values
Gross annual earnings
Share prices
This is how a clubs wealth is determined. The reason there not the richest club in the game is due to the colossal loan debt on the club which Forbes(im assuming thats who your referring to) dont factor into the equation.
I started a thread on exactly this. FM is wrong as it shouldn't include debt. Barca debt is just as bad Man Utd. If the game data on debt isn't right it shouldn't include it. The debt is serviced in clubs in different ways in different countries.
So I'm saying the debt shouldn't be a factor on the wealth of a club.
Take some owners from a club and they are nothing. Take Glazers from Man Utd and they are still worth over a billion.
Man Utd are worth a billion, in assets and value, but in debt on paper..
My house can be worth 100k, I'd still have just my wages in the bank... ¬_¬ lol