Because the bug happens when you're trying to get something done and you don't have 5 spare minutes to interrupt your flow in order to file a bug. You might make a mental note to report it later and then forget. Or, if the bug is annoying enough, you might just never use that program again and you don't care if it gets fixed. Or your wifi glitched at the same time the bug happened and you've no idea if it is a bug or the glitchy wifi. Or you don't know it's a bug until later and by that time you've no idea what you were doing to trigger it.

There's a million and one valid reasons people don't report bugs.