The juice required to cook rice would chew through batteries like there's no tomorrow. Even if large capacity Lithium Ion batteries are used, they need to be recharged and therefore plugged in. Also, Lithium Battery packs and water don't mix - leads to a rather impressive explosion with a large fireball (linky to vid: - n.b. this is the result of an overcharged battery, but the results are the same when exposed to water).
Bottom line - doable but uneconomical and with no benefitfor the user. Which means that someone will still make one and attempt to sell it.
I would never get a battery powered rice cooker..that would be so annoying. Luckily my rice cooker has a cord where I can adjust the lengths, so i do not have a problem with the cord being too short or too long.