I often ask myself questions about strange things. You know things like "why is the sky blue?" and so forth (drives my wife and kids crazy). One question I had years ago is why are kitchen wall cabinets typically 12" deep? After all is this the most practical size for wall storage? Those of you who have large dinner plates are replying "Heck, no!"
The answer lies in how the whole idea of built in kitchen cabinets evolved and the short answer is that back in the day when carpenters and trim carpenters built cabinets on site rather than in a shop or factory they used 1 x 12 pine boards for the ends and shelves.