This was not "Delta."

It should be reminded now that the real Delta is focusing more on international travel and chasing merger partners, they are outsourcing a lot of their more mundane domestic traffic to (mostly) inferior "Delta Connection" operations. Flight 4072 is SkyWest Airlines and despite the DL flight number, operates under a separate OC.

To the OP's point, yes it is a rare occurance of a lazy ground crew forgetting to load the gate checked luggage. The OP should make sure their complaint is heard by both Delta and SkyWest.