I have a number of valves around my water heater (in garage), but none of them will shut-off the entire water supply to the house.

Why do builders require home owners to go outside, remove a large metal plate covering the water meter in the yard, and then use the giant wrench and valve found there in order to shut-off the water?

Why do builders require home owners to go outside, remove a large metal plate covering the water meter in the yard, and then use the giant wrench and valve found there in order to shut-off the water?
They don't! You are referring to the water meter shut off. If you're on a slab walk around your house and look for a 3/4" pipe coming up out of the ground and going into your home.
If you have a basement look for a pipe coming in from the street. Both pipes should have a shutoff valve close enough to see. Good luck and thank you for rating my answer. Tom

Tom, I'm on a crawl space but the water enters my house below the garage slab, rises inside the wall between garage and house, and comes through the wall into a small closet inside the garage.

There are three valves there, but closing all of these does not shut-off cold water going into house. My house is about 20 years old.

At one time in my area it was required to have a water cut off just outside of the house where the water enters the house. This was called a stop and waste because after you turned the water off the valve have a small hole in the side that was suppose to drain the water from the lines in the house. A 3/8's square rod was used to turn this valve off and on, but the soil would rest off the iron rod and made the valve useless. Plus there was a question about cross connections.

So it is possible that you have one of these underground somewhere.

Another reason you may not have a cut off is a thing called gutting costs, there are lots of little things that a builder can cut on a house and first thing you know you are making another two to three hundred dollars off the house.

There are three valves there, but closing all of these does not shut-off cold water going into house. Then what do they shut off? 20 years isn't old for a house. I have a ingrown toenail older then that. Cheers, Tom