Because it's easy to do. If you want to make a small adjustment to the fore/aft position, you can measure to the center of the bars before and after the change.

If this measurement is to have any value, you need to use the same saddle if you have multiple bikes. Between different saddle models the measurement is only ball-park. I tried 4 different saddles last year and they were all within 1cm of the same, once setup to place me in the same position, but none were the same as my old saddle.

If you want real accuracy, you have to perform a KOP measurement with each saddle and even that is not easy to do with a great deal of accuracy. A person won't get on a saddle and sit at exactly the same location each time. Riding the bike on a trainer with the bike set level is probably the most accurate.