Screens and radios do a lot to limit battery life on most modern devices even if the energy use running the OS and user software was free.

If with a reasonable battery standby mode can only last a few weeks and active use is at best a few days then you might as well add a fairly beefy CPU and with a beefy CPU OS optimizations only go so far. This is why eInk devices can end up with such a noticeably longer lifespan, they now have a reason to put in a weak CPU and do some optimization because the possibility of a long lifespan is a huge potential selling point.