Interior ceilings are often built as modular panels for this reason, and sometimes even floors. But walls, especially exterior walls, are engineered to serve mu

Interior ceilings are often built as modular panels for this reason, and sometimes even floors. But walls, especially exterior walls, are engineered to serve multiple functions, such as acting as a moisture, thermal, and acoustic barrier, in addition to carrying utilities and structural loads. All of this is best served by arranging, and sealing different material layers carefully within the enclosure. Within the ruthless logic of cost optimization, incorporating modularity conflicts with building-code-regulated neccessities, and will only be used by some small percent of people, for some small percent of time.