its a type of resin wood that are used a lot in south east asia, it is used to seal the pot and also to seal ships and boat in boat buildings in south east asia, i dont know what they call in cambodia but in my country we call it DAMAR. Its like lacquer i think
I can only guess, that was what people simply observed in nature. Round egg, coconut shells, etc, good container. (Actually science learnedthat best energy containment with respect to strenght is sphere shape, and egg shape has good circulation) Hence maybe that what they also observed, that round/ spherical is stronger. Also easier to clean.