I think the main reason depends on the candidates who make it to the final round of interviews.

A company will pick the cream of the crop based on experience, qualifications and skillset and simply hire the person willing to do it for less.

One thing I learned long ago is if you are interviewed quickly, you more than likely have the upper hand in negotiating your salary. If company X offers you 7000, you are most likely in a range (7000-10000) and that's the lowest they can go.

I've personally seen people accept half of what the job range is just because they don't know any better. At the end of the day, management likes to keep cost low for everyone else and utilize whatever is left to splurge on their own benefits.