That doesn't work in all cases. I've recently come across two examples where we had a hard time explaining our costs even though we extensively tag and even have fine-grained AWS accounts:

• Some costs can't be tagged or at least not easily, e.g. custom CloudWatch metrics.
• For some resources it makes a lot more sense to provide them centrally for multiple services at once, e.g. NAT gateways or load balancers.