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https://www.verywellhealth.com/why-are-graves-dug-six-feet-deep-4047407

Where Did “6 Feet Under” Come From?
The idea that graves should always be 6 feet deep has been around for a long time. There is no agreement about where this idea came from. Here are a few theories about why people may have decided to bury their dead 6 feet deep.
The London Plague of 1665
In 1665, London officials issued a pamphlet they hoped would help stop an outbreak of the plague or Black Death. Some people think this was the origin of the 6-foot standard.

The Lord Mayor of London ordered that all “graves shall be at least 6-foot deep” under the presumption that doing so would prevent the spread of disease.7 Unfortunately, the pamphlet didn’t explain the reason for the 6-foot mandate.

There are a few reasons why this probably isn’t the origin of the 6-foot standard.
Between 1665 and 1666, there were an estimated 100,000 plague victims. Many were buried in mass graves called “plague pits.” These graves were sometimes 20 feet deep or more.
The orders also didn’t say in force long. This is because the outbreak quieted in 1666 after the Great London Fire. It’s not likely, then, that the “6-foot requirement” had enough time to become a tradition.
While it’s possible that London’s 1665 plague orders created the lasting impression that graves are always 6 feet deep, it’s not likely.

Gravedigger Safety
Some people think 6 feet was just a matter of safety. Deeper graves might need bracing to prevent cave-ins. This would be especially true if the soil was sandy.

Average Gravedigger Height
The depth could have also made grave digging easier. At 6 feet, an average-sized gravedigger could still toss dirt out with a shovel. He could also get in and out without a ladder.

To Prevent Disturbing the Corpse
Grave robbery or “body snatching” was a serious problem during the early 1800s. This was especially true in England and Scotland.
Medical schools in these places needed bodies for anatomical study. Some people met the demand by digging up fresh corpses.8
Cemeteries had a lot of ways to deter grave robbers, including:
Heavy stone slabs
Stone boxes
Locked above-ground vaults
Mortsafes, iron and stone devices used to protect graves
People may have also buried bodies 6 feet deep to help prevent theft.
There was also concern that animals might disturb graves. Burying a body 6 feet deep may have been a way to stop animals from smelling the decomposing bodies.

A body buried 6 feet deep would also be safe from accidental disturbances like plowing.

To Prevent the Spread of Disease
People have not always understood how diseases spread. During disease outbreaks, they may have feared that bodies could transmit disease.
While it is true that some illnesses like cholera and tuberculosis can infect people who handle bodies, this is not true for other diseases like bubonic plague.9
Still, this may be one of the reasons why people thought bodies should be buried 6 feet deep.

Folklore/Rule of Thumb
An old “rule of thumb” says graves should be as deep as the deceased is long. This rule of thumb has unknown origins.

In the 17th and 18th centuries, the average male was 5.48 feet tall. It’s possible, then, that 6 feet was just a good rule of thumb.10

You bury the ones you are about 6 ft deep, the rest in a shallow grave.