For obvious reasons. Almost everything can tolerate 5V. Very few things can tolerate 9V and above.
Consequently, you need a complicated dance in order to ensure that everybody agrees before jamming 9V, 15V or 20V down the pipe. That dance has enough complexity that effectively everybody uses a full-blown Cortex M0 with a couple of purpose built analog blocks to carry it out.
See: https://www.infineon.com/cms/en/product/universal-serial-bus...
If you look at Digikey AC-DC power supplies, you can find about 12K that end in a barrel jack.
Of those 12K, only 6 are not a 2.5mmx5.5mm, 2.1mmx5.5mm or smaller barrel jack. And Digikey seems to not even stock the mating jack for those 6 power supplies.
There is also absolutely no standardization of what voltage level corresponds to which jack. I have about a half-dozen dead Beaglebone Black's over the years because I had two different power supplies on my lab bench for testing and the plugs were "juuuuust" close enough (2.5mm and 2.1mm are compatible enough to let you short things out even if you can't completely seat them) to let me put 12V or higher onto a 5V-only jack.