Why do PUT/DELETE need idempotency but POST often does not?

HTTP idempotency

Idempotent methods: repeating request has same effect as once. GET/PUT/DELETE idempotent; POST creates new resource each time unless you add idempotency keys.

• Retries after timeout must not double-charge—use Idempotency-Key header.
• PUT replaces resource at URL—safe to retry.