Because inventing a whole new battery technology that has five times the performance at one-fifth the cost is a simple matter of throwing money at it.
And, while we're at it, why is anyone still building rockets? Why isn't NASA funding a crash program to develop the Bergenholm generator?
Whattya mean, "because there's no reason to believe it's physically possible"? What kind of excuse is that?
Improve, presumably, energy density by a factor of five. Let's see... I don't have the number at my fingertips these days, but for a primary cell there's not much room for improvement on yer basic lithium coin cell. I recall doing the math on that one 'bout twenty year back, when working on a project to develop a small electronic gadget that would operate for a decade without anyone having to be aware that it had a battery (and using 1993 technology). Once we ruled out hydrazine-air fuel cells and RTGs, it looked like we were stuck with lithium as a safe-ish, self-contained energy store.
Assuming that the intended new battery tech uses chemical energy storage, just how dense can that get, anyway? Another fun calculation, regarding single-use chemical energy storage devices:
AA alkaline cell, around 3AH * 1.5V * 3600 sec/hr = 16.2kJ.
.223 cartridge, 0.5 * 0.00356kg * (990 m/s)^2 = 1.74kJ.
Hey, look! Those checkout-line batteries have better energy density than smokeless gunpowder!*
So what sort of chemicals are the new batteries to use?
Or will they use some other method of storing energy? Warp the space-time continuum? Spin up tiny black holes? Force nuclear reactions to run uphill?
Is there any reason to believe any of this is possible?
Or is this just a new way of scamming funding, and anyone who questions it will be declared "anti-science" and "anti-environment"?
*Yes, this does mean that electrically-powered sidearms aren't obviously implausible; there's just the small matter of getting the energy out of the battery and into the projectile fast.