"That salary was what I was worth when I started working for them. I've learned things since then. If you don't believe I'll be more valuable to you than to them, why are you hiring me?"

which is a non-answer.

your salary often has no bearing on what you're capable of.

If I'm working for a company that only knows how to extract $80k of value from my labor, and then only pays me $65k, that says as much about their ability to manage their business and resources as it does about my innate ability. My exact same skills might bring $250k of value to a different organization.

Paying me based on what I earned in the past is ... wrong.

They have no way to ascertain whether I have improved or not. ;-)

Plus what I might be offering them may be very different from what I offered my previous employers.