The problem is the system on the other end usually has timing/causality expectations (e.g., in the case of reentrant calls), and that system is not usually part

The problem is the system on the other end usually has timing/causality expectations (e.g., in the case of reentrant calls), and that system is not usually part of your system (e.g., they wouldn't include whatever monotonically increasing counter your system would require to ensure causal consistency).