In practice, there is little if any difference between seals and signatures in tems of security.

A signature (or stamp) is easy to fake and get away with for a while. It's very rare that the authenticity of signatures is checked right away. Perhaps even easier than stealing or faking a not-particularly-secured stamp. It only happens when some problem arises and is investigated after the fact. The question is not whether the signature is "authentic enough" but who signed the document. You can aks and answer this question about a seal equally well.

The reason we have signatures (or stamps) is as an explicit ritual signifying ratification of a document that one cannot plausibly deny later.