I rebuilt a 6 cylinder Chevrolet car engine and decided to start it up before I had the exhaust manifold bolted on. It was loud, it ran, but I had to keep giving it gas to make it run. If I tried to let the engine idle, it would quit. I figured I would adjust the idle after putting on the exhaust but after the exhaust system was installed, it idled perfectly with no adjustments to the carb.
I noticed a similar phenomenum when I took the pipe off of a two-stroke motorcycle I owned and let the exhaust come straight out of the port. It didn't need the muffler to idle normally, it just needed the header pipe.
The most logical explanation for this is that when the throttle is closed, engines blow out some exhaust and then suck a little back in before the exhaust valve/port closes. Without a short exhaust pipe, the engine sucks fresh air into the cylinder resulting in a leaned out idle mixture. It may be possible to adjust the idle mixture to compensate for this.