You're only looking at it from the point of view of what you want. From the copyright holders point of view you are benefitting from all of their hard work, so they deserve financial compensation. It is someone elses property, and using it without permission is stealing. What if someone decides to use one of your video clips without your permission and makes a killing off of it; wouldn't you want to be compensated? The shoe sure pinches when it's on the other foot....
Now, you don't have to get permission, but there is always the possibility - however remote - that the copyright holder could come after you for copyright infringement. At the very least you would have to retain a lawyer, and that can get very expensive. Worst case scenario you end up paying out big bucks. I've heard stories; the best/worst of which was a very junior partner at a law firm who cruised YouTube and went after guys like you to impress the senior partners with his diligence.
Another way to look at it is that you are very talented, your vid goes viral, but all of a sudden it gets yanked because of the copyright infringement, killing the momentum and your chance for "fame and fortune".
BTW, there's another thread about just using video game sounds, and the answer is the same.
Can't you cut around it?