Because I already own one?
Because its the better caliber rather then my bolt gun?
Its a dull finish whereas my bolt gun is gloss?
Because I have a shoulder (or other) injury and recoil is softened in a semi?
To get attention in camp at sight-in,as most hunters still think semi's aren't accurate ?
Because my Grandpa & Great Uncles did after WWII & Korea?
To preserve the ability as an American to use any rifle for any lawful purpose?
I guess the reasons can and are indeed many as determined by the user!!
This.
Not to mention, semi-autos are generally more conducive to the mounting of suppressors, which I value. It minimizes flash and decreases noise, maintaining night vision and hearing (as I generally hunt without hearing protection, in my AO cougars and wolves are making a comeback). Also, my semi-auto 6.8 SPCII AR15 is lighter than most bolt- or lever-actions. Follow-up shots are sometimes necessary, nobody's perfect. I'm no tree-hugging hippie, but I don't want to watch an animal suffer. So, increasing the speed of follow-up shots is useful to me. If I can get another round on a wounded animal to end it's suffering. I don't take shots I'm not confident in, but again, I'm not perfect. I can't plan on every wind gust, sudden movement, etc. that might mar my shot.
As I mentioned before, cougars and wolves are bad news. While the likelihood of attack is very low, I know that with a semi-auto rifle, I can fire more rounds faster without breaking cheek weld, not to mention my Aimpoint is easier to follow-up with than a 3-9x42 or 2-7x30 scope.
Now, you're saying "that cougar/wolf pack will get you before you know it's there". Maybe. But I have multitudinous reasons to carry my AR already (weight, size, follow-up shot speed, etc. as I've already outlined). But, I'll take every advantage I can if I'm on something's menu (and yes, you are on a cougar's menu. If the wolves get hungry, you're on their menu too).
Just my mileage.