It's the length of an A4 sheet of paper (also, by extension, the size of many school books, backpacks and … rulers).

Why shouldn't a measurement that is so natural and familiar to most people that most human societies developed an analogous version and even places without it still end up using it, be a first class citizen of that measurement system?

That's what confuses me. It's a dumb confusion admittedly. It's basically an argument to "Beauty" or aesthetics or similar.