Why does Shopify charge a 2% transaction fee for third-party providers?

My primary payment gateway is Payfast, which most shops use in this country. Their fees are 3-4%, so you can imagine what a tough pill it is to swallow to pay an additional 2% on top of that to Shopify as well. As a matter of interest, Shopify alternatives like Woocommerce charge 0% transactional fees when using third party providers.