Firstly, rich people already do this with our existing currency systems. So that has to be what we’re comparing against. And nobody has done this because there’s zero benefit to doing so.

The thing you’re talking about is a 51% attack and the answer is:

• The cost of doing so, which continues to increase and is around a trillion dollars currently. Even if you had the money, there are very significant logistical hurdles which make it difficult and means people would see it coming a mile away. They don’t have to buy coins, they have to buy energy and equipment to turn that energy into mining and they have to keep buying energy as long as they want their attack to continue. That trillion dollar figure is for one block worth of attack (10 minutes). The longer you attack, the more the cost per block goes up too.
• There is no benefit to doing so. The second your attack ends, the network reverts to the true “main chain”, the system is designed to be really robust

There are only two things you can do with a 51% attack

• “double-spend” meaning you spend the same coins twice. But if somebody is going to trade you 1 trillion dollars of stuff, they’re going to wait for more than a few blocks confirmation. The scenarios where this makes any economic sense for anybody to attempt are basically zero.
• Delay (censor) transactions which will go through the second your attack ends

Even if you controlled 51% of the network you cannot:

• Spend money you don’t have the key for
• Increase the supply beyond 21 million coins
• Otherwise make invalid transactions

Because all other nodes would reject your transactions as invalid.