In all seriousness, you can't even add in leg kicks and call it fair. Boxers don't look for any of that stuff and can't undo a decade or two of training and incorporate something other than hand strikes. Can't defend or even watch for leg kicks, even. Boxing is a one-trick pony (hands) sport and while also not "fair", the other combatant is simply dialing down to hands only, not doing anything "new" which the boxer would be forced to in any other format.
But I know what you mean in that they are refined and so skilled with their hands that in a hands-only event, it makes it very hard to box a boxer for someone from another discipline(s).
It is what it is, though.