How about because it is simple, easy to remember & works well enough as a basic estimate for average conditions.Anchor scope - why do we teach beginners such rubbish?
Rather like the "rule of twelfths" which I can use to estimate depths (for crossing sandbanks or using swatchways) in my head while steering & looking at a chart.
Or the 2hrs(etc) before/after HW guidelines for harbour entrances & bars given in pilot guides.
They suit me & work well for my purposes. I don't want to be running round trying to find the right graph for each situation to measure my chain to the last foot; especially when I work with 9-13m tidal ranges.