@osma @m @benny @feditips @stephan This is not universal to AP, though (e.g. Firefish does import posts when moving accounts).
It's also not universal to what all users care about. The term "microblogging" notwithstanding, I expect a lot of users think of it closer to ephemeral conversation; posting something today and having some people read and reply to it is more important than whatever we posted five years ago. If I was posting something that I expected to still care about five years later, I would more likely post it to an old-school web site / blog, and then just post a link to it here.