The Gregorian calendar was introduced in 1582. It was finally adopted by the last major users of the Julian calendar in... the 1920s. So there's the issue of getting the entire world to agree to do something. How often does that happen? And you say "fix" but what's broken about a month of 28 days? What would adding 2-3 days to February gain that's worth the massive, world-wide disruption?

Though this does raise the question if the architects of the Gregorian calendar had more ambitious goals like equalizing months or changing other things that were later dropped. I wonder if anyone knows?