That's a little different because the pin pad might have an audit trail built in to it that would allow the merchant to prove (or at least provide evidence for) the provenance of the signature. I don't know if pin pads actually do this, but it's possible. With a signature that comes in as a stream of bits over the internet from some unknown source this is not possible even in principle.
And that is exactly the problem. People think it's the signature that matters because that is what they see. It isn't. What matters is that the signature provides some evidence about the intent of a particular person at a particular time. A physical signature on a physical document provides such evidence. An electronic signature does not and cannot.
BTW, even physical signatures have pitfalls. They bind only to the single sheet of paper they are actually on. This is the reason that on documents that actually matter they make you initial every page. Because without those initials it's trivial to swap one page of the document for another and it is impossible to tell which version you actually signed.