Actually, for things with a non-conductive casing (think cheap nightlights, or those apple wall warts) there's no need for a ground. If anything, it's another part and more cost, thus it isn't included.
In highly sensitive A/V systems, an isolation transformer is often used as well, neglecting a ground.
You have to be careful with isolation transformers because there is one gotcha... the ground pin normally passes through to earth. I've witnessed someone blow up a nice $5000 Agilent oscilloscope before I could stop them because they didn't understand how to use an IT and plugged the scope into it so they could probe AC. Bad news, you're supposed to plug your test device into the IT not the scope because the BNC connectors in almost all oscilloscopes are connected straight through to the earth pin.
I heard "POW" from across the lab as soon as he clipped the probe on and knew I should have warned him. Nobody was hurt apart from our lab budget.
I can't tell, the second boom where he holds the resistor in place was that on purpose? If so holy shit that's a dangerous way to make the point. If it was also an accident then he is lucky to be alive since he doesn't seem to know what he is doing.
I know this video is supposed to be funny, but I couldn't bring myself to laugh. Too many people die making the same mistakes.
He does know what he's doing and it certainly was deliberate. While most of his stunts probably hurt he doesn't set them up exactly as he presents them (I.e. it's usually not mains).
These are all better and more knowledgeable points than my own! I definitely only speak out of an abundance of caution having known people whose bad electricity and habits have damaged property.
In highly sensitive A/V systems, an isolation transformer is often used as well, neglecting a ground.