Because the engine is running. Shut it off or give it gas and it will not idle.
The engine is running C O C K
Keep engine running idle speed
If the starter is whining while the engine is running at an idle you have a problem with the ignition switch or the starter solenoid is stuck. The starter should not be engaged while the engine is running unless you are holding the key over on the start position.
Turn the idle adjustment screw on the carburetor counter clockwise while the engine is running, until the engine idles between 950 and 1000 RPM.
When an engine is in "idle" it is taking over by itself without any acceleration (or load) being applied. This the "idle RPM" would be the number of revolutions it makes per minute while running at "idle".
low idle and uneven idle
There are several things that could cause this. The first thing to investigate, though, is the idle speed your engine is set at. As with any engine, if the idle is set too low, the engine will not continue running.
Normally, applying the brakes. The engine may stop running due to vapor lock or a variety of other reasons, such as a blocked fuel filter, clogged fuel injectors, water in the fuel, etc. Also, low engine oil can cause excess friction, further causing the engine to stall while at idle. Another possible cause is low engine idle speed. If the engine speed drops below about 500 RPM while at idle, the engine will stall. Several things will cause a decrease in idle speed including strain from the A/C compressor, alternator, and/or power steering pump while turning the wheel.
No, an engine idle RPM being not zero is not a problem. If the idle were zero, the engine would not be running!
An engine usually has a higher idle speed when the ac is running. If the high speed idle does not operate then the ac will cause the engine to stall.
When the ac is running the idle should be raised to compensate for the extra load. If the idle isn't rising then the car will cut off.