Repeat after me: "Fill the space."
The secret to creating a look that looks like high-end built-in cabinets is to fill the space you have as completely as possible. These freestanding shelves almost perfectly fill the niches. They could have been purchased as single units, or could have been four separate bookcases in each niche. Whatever it takes to fill that wall or niche, fill it up instead of having a too-tiny storage unit stranded in the middle of a big wall.