at core, I think it's because twitter was 'public first', with some notions of private communication grafted on later (DM, etc). But the first and primary purpose of twitter is short form public posting. Many other platforms all try for social/friends/community first (or exclusively) and end having built-in limits.

No one worries if a tweet is seen by the 'wrong' people - it's meant to be seen by everybody. Facebook is the opposite - private/groups/friends first/only, with some notion of 'public' visibility after the fact.