The marketing of Cell Phones is such as to make it as cheap as possible with the features people want.
Solar Powered Cellphones exist, (http://www.mobileburn.com/news.jsp?Id=6369) and for just about any cell phone out there you can buy a Solar Charger. But the charging is not as fast as plugging it in and Cell Phones use a lot more power than a calculator so normal ambient light will take a couple hours charge a phone enough for a 20 minute phone call.
Outdoors in direct sunlight, it takes hours to get a full charge with a larger plug in solar panel.
And here’s the biggest reason - A significant portion of the market is turned off by “green” energy and doesn’t trust it for one reason or another. Someone will need to do it first and when it’s been around for a while, they’ll trust it, but until then one company is going to have to be willing to make the gamble. And just because LG and HiTech make Solar Powered Cell Phones doesn’t mean there’s a carrier out there that’s ready to make that gamble yet.