Why aren't all simple structs Boxed Types?

[murrayc t-online de (Murray Cumming)]

As I understand it, Boxed Types are non-GObject structs whose type
information is registered with the GType system.

So why are some structs (for instance, GtkTextIter) not Boxed Types,
even though they seem to have a similar set of new/copy/free functions?

-- 
Murray Cumming
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www.murrayc.com