My answer sounded condescending upon review. Sorry. I'll tell you a tale of a man who found out about bitcoin in 2010. This man thought it was impossible to have a decentralized currency because any digital thing can be duplicated. He had made thousands of copies of digital files before so how could this be any different. Yeah, that was me. I was ignorant. Eventually I learned.

The reason they can have as much mining power as they like is because as the hash rate increases, something called the difficulty increases along with it. In fact, the hash rate HAS steadily increased over time, yet the bitcoin protocol and rules are steadfast. The hashrate increases, it becomes more difficult to perform the hash. It's baked in.

If they tried to cheat and not abide by the difficulty, all the non-mining nodes would reject the blocks. They would fork off into their own chain that is no longer bitcoin and nobody would care or accept their transactions/blocks.

If you google the whitepaper, you can read how that works.