In general, the planes that do have mufflers are much lighter, simpler and less restrictive than a car muffler. You can’t hang a 20 lb car muffler on a plane where weight is critical to performance. It’s easy to hang a heavy muffler on a car and have it whisper quite with very little weight penalties. All aircraft manufacturers strive to reduce weight wherever they can. It means the difference between having a plane that gets off a shorter runway and climbs at a greater rate, than just struggling off the ground and barely climbing over the trees off the end of the runway. That means a safer plane overall.
Like porterjet said, many times the loudest noise you hear is the prop tips making that loud rasping or bark that you hear. Certain planes have louder prop noise,
like a T-6 which you might see at an airshow, or float planes which generally have longer props, which means those tip speeds make noise. Once you pull the RPM’s back a few hundred from the take off setting, the noise reduces greatly.