The requirement exists regardless of how hard or easy it is to implement. "Trustless" is a requirement because history shows that there are many circumstances where existing payment networks cannot be trusted. Payment networks sometimes refuse to do business with certain parties merely because it's not profitable due to bad credit, a higher-than-average chargeback rate, public relations, or other reasons. Even when the networks themselves are not actively antagonistic, they are vulnerable to political influence which may take the decision out of their hands.

Where trust is feasible transactions can be settled cheaply in separate records and not on the blockchain itself. The Lightning network is one such protocol; support for inter-account transfers on the same exchange is another. However, it's good that the trustless option exists for the cases where trust would not be justified.