It's how they've been interpreted forever on Unix and other old systems

It's probably the main reason why they're so controversial.

Which was influenced by tab stops on typewriters... which were adjustable, but were generally set at 1/2", 3/4", or 1"... which was the equivalent to 5-12 characters, depending on your font (monospaced, but sometimes 10 cpi, sometimes 12 cpi).