Almost all imported cabinets come to the United States RTA/knockdown (ready to assemble). This maximizes the number of cabinets that fit in the shipping container. The cabinets are then assembled in the United States. American Made semi-custom cabinets are built to order, meaning no wood to make cabinets is cut until the manufacturer has your order.
Because of limited colors, wood species and SKUs, imported cabinets are generally cheaper. Importers are focused on stocking the most popular colors and sizes. For instance, stacked wall cabinets are very popular in kitchen designs. Stacked wall cabinets built using imported cabinets will be limited in available sizes, widths and heights, while a domestic manufacturer can build to order in many more sizes, wood species, and colors.
Of course, there is an added cost to offer this flexibility. But it gives the kitchen designer much greater flexibility for their designs.
Pantry cabinetry is another area where domestic made cabinets offer more options. Many importers offer 18″, 24″ and 30″ wide pantry cabinets in a variety of heights. Where a domestic semi-custom manufacturer will offer pantry cabinets starting at 12″ wide and going to 36″ wide cabinets in 3″ increments.
There are many more ways to compare the two, but this gives a general understanding.