Exactly. And what about eye gouging, groin kicking, all sorts of other illegal stuff that can happen on the street?
I remember some story of Ken Shamrock who had a habit of playing Robin Hood trying to get into it with someone (back in the day) and got seriously injured because the guy was just a crazy thug, experienced street fighter and criminal with nothing to lose.
This is not a Jean Claude van Damme movie. MMA is a sport with certain rules and a ref. Boxing is a sport with rules and a ref. You have no idea who you're dealing with on the street where anything can happen.
And even assuming that it's strictly one on one -- that's not a real fight. Because in a real fight their buddy could jump in, he could have a gun or a knife, etc.
I've seen that kind of stuff happen when I was younger so even assuming that it's strictly one on one is already assuming some kind of rules so that's already a limitation.