WHY IS A GAS OR DIESEL GENERATOR'S POWER LESS STABLE THAN THE GRID?

When power is generated from a power plant it isn't necessarily stable at that point. In order to stabilize the electricity which may run at anything from 1000 volts or above (but usually it is 1000 volts when traveling on power lines to your home in the U.S. It can be 50,000 volts for long distances but then has to be reduced to 1000 volts in a substation near your home within 10 to 25 miles usually.
It is sent out at 30,000 to 50,000 volts to prevent line drop which occurs at lower voltages. Line drop just means you get less electricity at the other end of the line that you started with.

So, sending electricity at high alternative current voltages and amperages solves this problem.

So, usually it comes down your street reduced down to 1000 volts which would be fatal to most people if you picked up that line if it is sparking and dropped by a tree or the wind. So, NEVER pick up 1000 volt lines for ANY reason because it is often fatal.

I suppose if it was killing someone and I was present I would possibly pick it up by the shielded part of the wire but not the part arcing and spitting molten metal pieces in every direction. I would pull the arcing parts away from the human who likely would be dying or close to dying then.

However, I have one friend who has been hit with 1000 volts and lived. He was blown 10 feet away from the arcing and woke up an hour or two later after being unconscious.

But, it's when your muscles don't blow you away from the arcing that you die. You get cooked like meat and then it's over.

So, what stabilizes the electricity to your home is the transformer on a power pole (usually within a few homes of yours or if you are remote you will have your own transformer on a nearby power pole.

Then it comes into your home as 220 volts in three wires. Two of these are hot and one is the neutral.

So, if you want only 110 volts all you have to do is to take one of the hots and the neutral and you have 110 volts. But then you have to have a circuit breaker for safety in case of emergencies so it doesn't burn your house down if some kind of short circuit happens. When the short circuit happens your breaker or fuse is tripped so you don't burn your house down and hopefully no one dies when you get a short circuit either.

That's why you have a main circuit breaker and then a breaker for each circuit in your house.

This is how you use an incredible force like electricity and keep it safe for use by children and adults.

Note: After all that I realized I didn't really answer the title question.

The real answer is that the changing speeds of the Gas or diesel engine varies the actual amount of voltage going down the line. So, 110 volts is an approximation of what you are going to get from a gas or dieself generator. This is why when using a gas or diesel generator you want a surge protector to prevent damage to any or all of your devices from use with a gasoline or diesel generator.

To have a perfect 110 volts stabilized all the time the engine could only run at one specific speed which would generate that exact amount of 110 volts all the time. But, with strange gas or eccentricities of generators that isn't going to happen all the time. So, once again you want a surge protector to protect your devices from refrigerators to cell phone chargers from increased or decreased line loads. Because also the amount of things you are doing with the electricity from the generator also affects the amount of voltage traveling to your devices in any given moment.