One of the reasons why is because we don't have full documentation of some of the hardware on the later iPods, like the 4G/color, nano, and video. There are certain parts of the hardware we can turn off to help conserve power, but without datasheets, the best we can do is just reverse engineer.
This doesn't just affect the iPods, but also other PoratlPlayer targets: the iriver H10 and the Sansa c200 and e200 series. There are some parts of the PP hardware that we haven't figured out yet.
On a battery life note: my iPod video gives me about 8-9 hours of battery time on a full charge. That's on regular music usage, and I think that's not too bad. I'd certainly like it to be more, but the only to make that happen is I have to go out and look for information about the PP chip if at all possible.