Someone already mentioned cache invalidation. To extend that, I don't think it's all that different from an old paper system. If you want to delete your file it's probably kept in a cabinet in some department, but the billing department or marketing department also has a copy of your name and address in their records. Deleting everything is a multi-step process.

Centralized systems didn't scale in the physical or digital world and it distributed systems complicate things that seem trivial.