I spent a fair amount of money through Protolabs and Xometry at my last job (mid-five figures a year), but only because they refused to bring any real machining in house. Drove me nuts. I could not get the price and delivery they could promise, and my quality requirements weren't onerous. The biggest problem I had there was actually with one of our vendors for production parts. He lied about checking some parts and they still placed another order with him. It's one thing to screw up and say you're sorry, but that was the last straw for me and I never asked him to quote another part.
To return to the topic at hand, I do wonder what's in it for job shop owners. The service takes its cut for a price and delivery that are both less than most shops would normally charge. Maybe there's a way to tool up to make that a profitable venture, but I'll be honest that I don't see how.