Not strictly true, Ghost has a lot of built-in features for SEO. In particular: Detailed schema.org microformats and structured data, automatic canonical tags, automated + user override-able custom meta data, automatic XML sitemaps, SEO friendly routes, RPC pings, automatic Google AMP support, and quite a few other things.

We do all of these things in core, rather than leaving it up to the theme, so that everyone gets consistently good SEO rather than being left up to the mercy of whatever the theme developer remembered to implement