Because some fraction of people are cheaters. Just think of how many people you know who cheated in school. I'd say the number is 10% or higher. Some of those people became CEOs who continued to cheat, but many became low-level bank employees who continued to cheat.

And if you're being measured on how many accounts you create, and the system doesn't have checks to keep people from opening accounts without consent, it's not exactly hard for all the cheaters to figure out what to do. Again, think back to all the ways people cheated in school. You think they had to coordinate to figure out how to cheat?