The top response is interesting that you can't make claims based on specific hardware without knowing the specs. Point taken.
I used 10% to show what 10% would do for the example, I would be surprised if many mainstream ones are 10% lower. But we're talking about a market where people jump on a combination of super cheap and super light.
12oz, $20, same power specs as two NB10000. The NB weights 85% as much for two but costs $100
Light and cheap comes with less robust batteries, less robust circuitry. Do you really think they didn't cheap out on the boost conversion circuitry to save money if it's that light? Something had to give in the equation and it's probably everything inside it.
For the sake of your point, would you rather spend $100 for the same thing or try an adaptor and see if it gives you more battery life and you don't have to stop as often?
And say you bought a battery bank from that brand, that looks more like this (I know it's a better brand, just look at the port setup)
Guess what you just gained with the adaptor, the ability to carry all USB C cables for input AND output.
Doing more with more and now your input and output match