Why CodeIgniter not using native session?

Greetings,

I start using CI when version 2.1.4 launch, soon i realize that, CI session library are using cookie(which store in client browser).

The question is, why using cookie?

Are cookie have many advantage than PHP native session?

Back into my collage ages, the web development bible always said, never trust cookie because it will cause security problems.

Until version 3.X, the session driver providing quite a lot of choices but no native session
is there a reason?

Hope development team can explain.

Thanks.