Bitcoin is just software that everyone runs, and the rules of that software say only 21 million coins can ever exist. Every computer in the network checks that rule, so if someone tries to create more, the rest of the network rejects it. To change the rule, basically everyone in the world who uses Bitcoin would have to agree, and since it would make everyone’s coins worth less, nobody has an incentive to do that. That’s why the cap is unbreakable.