>Why are chargebacks higher?

People pretend their purchases are fraud or regret their purchase later so they chargeback more frequently than other services.

>Why can't that just be accounted for in the processor's rates?

Because it's easier as a credit card processor to just drop all of your high risk clients and focus on serving the rest of your customers as best as you can. Of course that leaves a whole in the market for other credit card processors that charge more for allowing higher risk purchasing.