It's more of an "end around" situation than one might think.
1) code says conductors must be rated for 125% of maximum continuous load.
2) inverse of 125% is 80% (1/1.25)
3) since the conductors sized for the circuit are over sized, that means the circuit cannot be allowed carry 100% current continuously anyway.
4) Since the breaker is there to protect those conductors, and they can only be loaded to 80% continuously, no need for the breaker mfr to design for heat dissipation at 100% load when mounted in a panelboard with other breakers, which is the difficult (translate; expensive) part for them.
This is why you will see that 100% ratings on breakers are usually restricted to being used as stand-alone breakers or main breakers, where there will be no other breakers directly adjacent to them. It's really the exact same breaker*, often just with different lugs that are rated for 90C, but not allowed to be feeders in a packed panelboard. All breakers are bench tested per UL standards at 100% current, it is the APPLICATION of them as feeders in panels that indirectly creates the "80%" rating.
* You will be hard pressed to find an employee of a breaker mfr who will state this, especially in writing. I used to work for Siemens, we were SPECIFICALLY trained to never say this, because it can lead to over interpretation and misapplication, then lawsuits where the sharks look for the biggest bleeders.