It isn't just about raking, edging, hand weeding (grass and other things will try to grow in every square inch of that bunker, not just the edges), checking depths, adding/subtracting sand, animal footprints, golfer footprints, fixing washouts, pumping them out after rain events, arranging rakes, etc.
The faces of bunkers are also a huge pain, as they are a frequent dandelion/weed hotspot, their pitch often necessitates increased irrigation via additional pop-up zones which require constant checking and adjustment. Additional fertilizer must be applied with a walk spreader or by hand, as the sand splashed growing environment is nutrient poor. Burrowing animals love bunker faces, and like to dig native soil out into the bunker where they make their homes. So you need to literally sprinkle edible poison such as oat bait on the bunker faces with burrowing animal activity. Any steep faces need to be flymowed, which is a very labor intensive process (not to mention dangerous), and then the clippings need to be blown out of the bunker before raking.
Furthermore, the presence of bunkers affects other general maintenance practices too. The more bunkers you have the more elaborate your rough mowing scheme must be. The big rough mower has to join up with the smaller rough mower which has to join up with the walk mower, string trimmer, fly mower, etc. Similarly, the proximity to greens can influence whether a green can be sprayed with a riding boom sprayer or a walking spray hawk. Spray hawking will double to quadruple your labor cost for that particular operation.
Tl:dr version: Build more Sheep Ranches.