1. Reason #1: Magazines can be quickly swapped, even before they are empty (i.e. when convenient for the shooter) such that whether a shooter carries one 30-round magazine or three 10-round magazines makes no difference. Check out USAMU member Travis Tomasie:
2. Reason #2: Magazines are based on the exact same design as a simple PEZ dispenser. It is very easy to either modify an existing magazine or make a new one into a "high capacity magazine." If someone set on mass murder cannot buy one, he/she will just make one.
It seems to me that most people who propose bans on high capacity magazines don't really know what a magazine is or how it functions. So hopefully some of them will read this. If as a kid (or adult) you ever ate PEZ candy with PEZ dispenser, you need to understand that firearms magazines are pretty much designed exactly the same way. The website collectingpez.com has this diagram of a PEZ dispenser taken apart (and what kid didn't take his/hers apart?):
And here is a Hello Kitty PEZ dispenser in action:
3d Printer" (an inexpensive home CNC machine).
Most firearms magazines are no different. Here is a diagram of the internals of a Glock handgun magazine: