Idempotency Keys for Exactly-Once Processing

Here's what I don't understand about distributed systems: TCP works amazing, so why not use the same ideas? Every message increments a counter, so the receiver can tell the ordering and whether some message is missing. Why is this complicated?

Failure resistant systems end up having a bespoke implementation of a project management workflow built into them and then treating each task like a project to be managed from start to finish, with milestones along the way.