"He who marches out of step hears another drum." ~ Ken Kesey
In defense of cans, they have their place. They chill really fast. While camping, a soak in a cool stream does not take long to reach the good stuff inside, and if one has access to to a shallow bowl and a few ice cubes, and a little water, a can be chilled in less than a minute, spinning it in the icy water.
Well, it's not that you necessarily suck at cutting, but like so many other things, experience can count for a lot. Some folks can follow a straight guide line better than others, but that can often be fixed by the use of a correctly placed, clamped wood, plastic, or metal guide to which the side of the saw can ride against.The other common problem with accuracy to newbie saw-ers, is not compensating for the kerf (the width of the material we now see as sawdust or metal shavings)Instant edit since last post:Mike has the same idea with his "fence"