This makes no sense. Who's more capable of keeping a solution for this developed, tested, and up-to-date, the core developers of the database or a third party that's probably hoping to achieve something other than a migration tool?

It's worth noting that it's not like rewriting the whole table is a solution to this that's unique to SQLite. MySQL's two main table types also do this in order to alter a table in certain (very common) ways. But they don't make you do it yourself, because it's deceptively difficult to do it right.