Not really a market gap.

Hit Amazon.com and search for "raspberry pi UPS" and you'll find plenty of options. Or more generically try search terms like "mini ups" or "router ups" or "dc ups" - loads of options at places like Aliexpress as well.

Here in South Africa with rolling blackouts happening fairly regularly these days a lot of homes have something similar to keep internet connectivity live when it happens, with a lot of those options having USB outputs as well.

I'm currently using one of these to keep my pi 4 + hdd setup online: https://acconet.net/product/ups-rp7500a/