Avionics shops these days are highly trained, competent folks that do an amazing job installing aircraft avionics systems. Panel design and machining technologies are but a side-step to the avionics industry, therefore most shops are not well trained with how panels are manufactured Often, they lack the personnel, training, and equipment to properly facilitate a custom panel installation- so they usually choose to outsource. During the process, they have to draft up the design by hand, and have the panel cut to what are often incorrect dimensions. The panel is usually still utlized, so they "make it fit" by modifying it by hand. They send the panel out to paint, and have legends made up by stickers or by silkscreen. These are all incremental additions to the overall project schedule as each process may take several weeks, and in the end your panel might not look as good as you hoped.Bottom line- your aircraft is usually down longer.