Why can’t cellphone companies come out with chargers that stop using power when not charging a phone? (Note to anyone who still doesn’t know: Chargers still drain power even when a phone isn’t charging.)

A recent report from some big major cell phone conference in Madrid has introduced the notion of the "universal charger" which will do exactly as you say. So far, the major companies are committed to it - a charger which uses a mini usb port and uses less power when not actually charging.
Unfortunately for us, but typical of the phone manufacturers, it will of course require us to buy new phones to use them.
As for me, I use a solar charger.
http://www.cbc.ca/technology/story/2009/02/19/tech-cell-charger.html

If anything is plugged in, it will drain power. -Note to author of questioner

Not just chargers, all appliances when on standby. Unless the plug is switched off.

There were a couple of people on Dragon's Den last year that developed a product that could do just that. We are currently waiting for it to fully come to market intergrated within appliances.

They will, as soon as more people like you speak up and say that's what you want, and that you're willing to pay more to save that little bit of electricity. Someone is going to have to research the best way to design such a charger, and that cost will be paid by the people who buy the phones.

I'd guess because it's too expensive per unit. It's probably a difference of having an adapter which is just some fine copper wire wound around vs having a little internal circuitry that would do as you describe; a spool of wire is much cheaper to the manufacturer.

Capitalism I guess doesn't necessarily promote the best ideas, often it favors the cheapest ones.