That's a pretty weird reading of the concept of the gig economy. Upwork clearly wants to own the customer relationship, which is why they do what they can to avoid being cut out of the deal. In a world where Upwork would be happy to be compensated by freelancers invoicing customers directly for work found through Upwork you'd have a point, but that is not the world we live in. Upwork processes the payments, and both clients
and freelancers are aware of this and that Upwork takes a cut for providing the matchmaking and process services they offer.
Service not delivered is normally dealt with by services like Upwork through escrow like arrangements or review procedures where the customer will only pay for approved work. Upwork chooses the second method, which means they have all of the control they need to ensure that freelancers only get paid if they deliver, substantially reducing the risk of such chargebacks.
And if these do happen, then the problem, again, lies with Upwork and the fact that they choose a payment method that offers that possibility. They could choose wire transfer instead if they were concerned with customers charging back after approving the work.