How come consumer products aren't powered by power banks?
There are two ends of the spectrum to consider: low discharge and high discharge.

For low discharge stuff (clocks, kitchen scales, smoke alarms), it does not make economic sense to use rechargeable batteries at all because they have high self-discharge; non-rechargeable ones can be used with a lifetime of several years.

High discharge stuff is less common but tends to already have proprietary power packs - drills and other power tools. This probably could be standardised across manufacturers but they don't want to do this since it would eliminate their margins on extra battery packs.

There has very recently evolved a middle range of Chinese-origin appliances that use 18650 rechargeable lithium batteries - mostly e-cigarettes and torches. Here the understanding is that you buy a bunch of commodity cells and use them across your appliances with a single wall charger, ie not putting any electronics in the cells. You can get an 18650-to-USB5V "powerbank" device too. And here's the 18650-using portable speaker you asked for.

An important thing to understand is that connectors are often surprisingly expensive - that's why appliance battery holders are simplified to a piece of bent metal and a spring.