Think of it like buying a used car. You go to the lot looking for a reliable car at a fair price. The dealer shows you a car that meets your requirements and adds, "and we're selling it for 50% below the blue book value." You'd probably be suspicious. Is it really what he says it is? Maybe the previous owner didn't get regular oil changes and the engine is corroded. Maybe the brakes are no good. Maybe the battery is about to give out.
Hiring managers want smart, reliable employees that will do the job they're assigned and add value to the company more than they want to save money on salary. This is why I believe it's always better to sell yourself in terms of what you will bring to the company and leave salary negotiation for the very end, after they've already decided they want to hire you.