Just a guess...
WAAAAY back in the day when data was slow and possibly expensive (read: those that did not have some sort of unlimited plan), syncing historical data could be an issue. Yeah, in theory, it's a one-time issue (eg. most people are not wiping their devices on a regular basis), but...
See it akin to how there has always been a limit on the size of a download from App Store you could do.
Or: can't recall if on by default, but know people that maintain their calendar history for ages, just like their browser history. For example, keep a meeting event from two years ago in the calendar, just in case they need to recall when that was. If on by default, can see some people getting a surprise when their old events are not showing.
Or: logic never worked, especially now with iOS 13 Reminders, where they are going all-in with folders, sub-lists, items with no dates on it (other than create date). Maybe accidentally excluding things that should not have been.