Capitalists stay rich and keep getting richer because of how the M-C-M cycle works. Back then, people like farmers would sell something to get money, then use that money to buy what they needed, so money was just a step in between. But capitalists do it backwards. They start with money, use it to buy stuff, and then sell it to get money back, but the money has to be more, and that extra bit is the profit. That extra money comes from paying workers a wage that only covers enough for basic stuff like food and rent. Like, if that wage only covers about 4 hours of the workday, but the workers work the full 8 hours, they keep making things and creating value during those extra hours without getting paid for them, and the capitalist just keeps that as profit. That is why the wealth never stops growing, because once the capitalist gets that extra money, they put it right back to buy more materials, hire more workers, make more products, and sell for even more money.