I can bet because stocking an inventory of batteries is a pain. The platform is constantly changing these days. There's no standard platform and they'd have to buy thousands of them to provide a cost effective package.
A low cost sine inverter using a cordless power tool manufacturer battery platform is quite affordable these days. No need to license these cordless tool manufacturer's platform as they provide the batteries and the inverters from their own line.
And UPS with Lithium-Ion batteries are also lightweight and cheap now.
Use one of these to power your low power amp head.