I will go ahead and ask another question: why does it need built in battery?

In the past 15-ish years since I have been using wireless mices, I have had only 1 that used up the battery in less than 4 months (incidentally I'm using it now, $4 garbage) - everything else was tagging along without a hitch for about a year, without turning them off. And when the battery starts to die, they just start blinking like a week ahead... I think mouse is one thing where replaceable batteries make a lot of sense still.