It's being tried all the time, the difficult part is that the gun produces gas pressure up to 55,000 pounds and it's difficult to control what's left when it escapes. An internal combustion engine is far far less, and the multiple cylinders allow having the pulses counteract each other.
But - a car is far from completely silent. Listen to a car rolling down hill and you will hear treads slapping on the road, the driveshaft working, and brakes making noise as they are applied. Much less automatic cooling fans under the hood, etc.
Same with a firearm. You could get the report controlled to a point, but what would be left is exactly what you hear charging the first round. All the clank clank and banging the action always has would become the next loudest thing.
"Silencers" don't, they just muffle the report. The best silenced guns have a closed locked bolt manually cycled to keep the noise down. With all the work around to get one that silent, it then becomes a matter of what might be a better method, knife, poison, or garrote. Some prefer a simple push as the subway tram enters the station.
Aside from that, electric cars come close to being silent, which is why they are required to play engine noise from a speaker so the blind won't step out into a crosswalk as they pass.
That same thinking might be behind the anti suppressor laws, we need to know exactly when and where somebody is shooting!