"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?"

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.

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.