If someone jumps you, you may not have a hand free to rack that slide. By the time you realize you need that gun, you may already be fighting for your life. Self defense isn't necessarily a whole lot like standing at the bench on your pistol range and calmly firing rounds into a paper target.
A fair bit of the practice I do is in shooting from "retention" positions, and not having two hands on the gun. (Usually having the support side arm up to ward off a blow to the head from my attacker's right hand.)
Chances? Well, if you feel that the statistics about needing to use a gun to defend yourself should guide you, then you may find it ludicrous to even carry one at all. And you couldn't be said to be "wrong" (unless, you know, you're wrong and someone attacks you). But if you look around the world and observe all the police officers and other folks like concealed carry practitioners who keep sidearms with them daily, you'll realize that something more (way more!) than 90% of them have a round chambered when the gun's in the holster. Chances are, they see something you haven't.
So, now you understand.
A million years sure passed in a hurry, didn't it?