If a person were to commit fraud they would do it on big, high value transactions. Fraudulently buying a pizza isn't really all that much payoff for our fraudsters after all.

They will buy stuff that's expensive. This means that to credit card companies, the bigger the transaction, the more likely that they will have to spend time and effort on fraud investigations etc.

Man hours are expensive though, and as such it is important someone pays for it. And well, that's where the percentage system comes from.