For those who don't know: The original definition of a metre is 1/40 000 000th of a full circle of longitude of Earth. That was considered not accurate enough, so it was redefined as the length of a certain metallic bar kept in Paris. Then it was defined in terms of wavelengths of krypton light. Finally, it was redefined in relation to the speed of light.

But each time the metre was redefined, the new definition was within the error bounds of the previous definition and the instruments that could be used within the previous definition - this ensured that backwards compatibility was retained. That's how we end up with these weird-looking numbers; it's not for fun and games.

Meanwhile, backwards compatibility was absolutely broken many times in traditional and imperial measurement systems. Heck, we have a break even in recent history: The survey foot has been discontinued in terms of the international foot, but they differ by 2 parts per million. That might not sound like much, but if you're measuring a whole continent, then being wrong by 2 ppm over 3000 km means having a discrepancy of 6 m, which is more than enough to fit an extra house in.