How many non-techie people do you think actually know the voltage of a USB port? Or that anything which charges from a USB cable will work with any charger with a USB port on it? I know a lot of people who know absolutely nothing about any of it, and can only charge their phone with their phone's charger. They would have no idea that 5v is the standard or that another 5v charger with the same physical connector would also work.but nowadays that is bullshit...
it's usb standard... the only difference is how much amperes it outputs (which translates in faster or slower charging speed) voltage is always 5v (except in special cases)