Because it's not an open standard any only support low-power devices. If we gimped USB-C to only support 5 volts, it would work everywhere too.
iPhone fast charging over lightning runs at 9 volts.
... and uses USB-C on the other end of the cable
There's no cable that has lightning on both ends; the 5V configuration has USB-A on the power supply side. My point is just that that lightning connector isn't limited to 5V.