if JESUS was buried above ground in a cave. with a roll away boulder. why are people buried in a coffin 6 feet under ground. REALLY?

I believe the man called Jesus was put in a tomb that belonged to someone else. Tombs are still used today but, they can be very expensive.

The latter is much more cost effective than the former.

Not enough  caves to go around? 

Because the burial customs of the first-century Judaeans

were quite different from our burial customs.

Part of the reason for that

is the very rocky terrain in and around Jerusalem.

Tombs and ossuaries (look it up) were the custom of that region for centuries

as opposed to trying to dig up the rocky soil

or establishing a large cemetery on relatively rare softer soil that was arable and used for farming.

The practice of burying in a cave was to allow reuse for future generations. The body is placed in the sepulcher for about a year. During that time, the bugs eat all of the flesh, leaving only the bones. The bones are then collected by the family which ceremoniously saves them. Then the sepulcher is re-used. Burying under "six feet" avoids the process of messing with the remains. Today, instead of bones, the remains are cremated and some people keep the ashes on the family mantle. Or scatter the ashes.

Different customs. It's forbidden for a Jewish priest to come within four cubits of a grave, so they're marked with large stones.

Because if I'm not mistaken, ancient Jewish burial customs required the body to be tended to and moved several times - from ritual washing to placing the bones into a box called an ossuary. In order to hasten this decomposition into bones, the body was left on a bench in the tomb to desiccate. This custom would be very difficult to perform if the body was buried in earth.

It was convenience for them to gather the bones later and place them in a smaller box for keep.

There are supposed to be at least 15 people buried for every person who's alive. I think that number sounds low, but that's what "they" say.

I used to go to Natural Bridge Caverns when I was a kid. Now I'm imagining that place full of bodies and it's not the most pleasant thought.

We tried keeping dead people around back in the day but they got pretty gamey very quickly.