Two reasons: you're not paying attention, or you're doing it wrong
A miter joint is any joint between two boards—or other building material—that form an angle, where each piece is cut to an equal angle, which is half the angle of the joint. Typically, this refers to two 45-degree miter cuts forming a 90-degree angle.
Unless we're talking about remodeling...
Step one on the road to perfect miter joints is to make sure your saw is dialed in to cut accurate angles, though Jonathan is pretty quick to point out that usually, the problem is not with the saw.
Reason 1 Why Your Miters Suck: Operator Error
One common reason, he says, is that carpenters will not support the ends of the stock adequately, meaning that the stock does not sit tightly against the fence and/or the table of the saw. Raising the bottom of the molding above the saw's table just one-sixteenth of an inch can throw the cut off a couple of degrees.
Same situation if the board is not pushed tightly against the fence.
Result: Your Miter Sucks.
Reason 2 Why Your Miters Suck: You are cutting the wrong angles
If your saw, your saw setup, and your saw technique is right, then the problem could be with you. Operator error—you are cutting the wrong angle. Just because it is a corner does not mean it is a 90-degree corner. In fact, it is almost certainly NOT a 90-degree corner.
Get a miter gauge/angle finder and USE it to measure the angle of the corner that you are mitering trim for.
These are simple mistakes that are simple to correct. If you correct them, Jonathan has complete confidence that you will get it right this time.