Well...
It is a huge longshot, and most of it is for the reasons that responders here have cited. However...
One of the biggest reasons is that the upper levels of hockey, and to a lesser extent, some lower levels, are clubs...and if you ain't in them, you aint' in them.
People generalize about NCAA d1 and D3, major junior, USHL, NAHL, etc, but the fact is there are PLENTY of guys in those leagues that frankly aren't any better than a lot of guys that don't make it there or beyond. That's a fact. Just the way it is. It ain't always about ability folks, in fact, a good percentage of the time its about politics and money and probably a few things no one wants to talk about. Whatever the case, lots of talented guys never develop past midgets or even Jrs and NCAA because they don't get a shot for whatever reason.
Sometimes, they're just young kids that start to realize that other things in life exist that don't involve getting their heads bashed in....chics, beer, rock n roll, pot, cars, cushy desk jobs, sandy beaches, etc, etc.
But, go tryout. At the very least its the chance to test yourself and prove you ain't near those that make the squad. Hey, maybe you'll get the chance to kick some ass too, that's ALWAYS fun and worth the trip. But making the team? Very tall order. Still, if you're young, in shape and want to try, it CAN happen. Just bring a lot of ibuprofen and butterflies and band aids in case.
Honestly, why would you want to? They don't pay much, its dangerous and getting to the next level (AHL or NHL) is as infrequent as winning powerball tickets.