There are graves in NO. Some people, instead of a family mausoleum had raised burial plots. And there was a Jewish cemetery near the corner of Elysian Fields and Gentilly (I think - it's been awhile) that didn't use mausoleums at all.
Yes the water table is high, but they even did mass graves during the yellow fever epidemics.
All true. The "cities of the dead" are not the only burial practice evident in New Orleans, though they are certainly a fascinating piece of the city's history.