Why is that? Because most developers look at the issue they have to raise and the code they have to write and figure it's easier not to bother.
But, whether that's the right thing to do depends on your process. Do you have a QA team? Do you think they mind if you just go changing code which won't be tracked? What about code-reviews? Will it skip by that crack? What about the business? Do they need to know you've fixed a bug so that they don't raise the same one later?
What about other developers? What if they fix it in a different way at the same time? What if they find a similar bug later and all you can do is say "oh, damn, I know we've had something like this before -- now what was it?"
There are about a million reasons for recording bugs in the bug-tracking system. If you're SURE you don't hit any of those issues then by all means, don't bother. But if you're at all unsure then you should record it, even if most people don't.