I found out why the lines were buried ! It was to quieten the lines . That's what the home owner told me.
Yes the lines will be very quieten underground
But otherwise it is common because of remote locations, sidewalks and driveways. Have serviced systems where the CU's were 100 feet off the house (customers didn't want to see or hear those noisy, ugly things). There were problems with that distance though. Very long time for pressures to stabilize. But hey they got what they wanted
Depending on the climate and length buried, lines in the ground can provide a place for liquid refrigerant to migrate to during the off cycle. If they are buried right before entering the condensor this can cause problems with the compressor.