Because some folks can only afford one shotgun that has to do double-duty. You can't carry an 8-shot 870 into the duck woods. The Game & Fish folks frown on that.
A gunfight is not the time to learn new skills.
If you ever have a real need for more than a couple of magazines, your problem is not a shortage of magazines. It's a shortage of people on your side of the argument. -- Art Eatman