This is intentionally bad itemization to encourage people to trudge through content using poetics gear but ultimately aim for gear from final coil. It's more noticeable for some classes more than others, but it's there. If you notice the gear with "good" stats for the most part comes from final coil; as a tank I end up swimming in parry, for example.
That was more an oversight that the developers did nothing about. My dislike for swaps was known to those who played with me, and I can say it really soured my outlook on XI's combat design.
Because the ends achieved through gear is to reach whatever stat cap or threshold that needs to be reached in order to progress. BiS as a concept is VERY different from things like the lv51 elemental staves. The former is part of progression, plays into your character growing more powerful and is tied with current content. The latter involves items overbudget for the level at which players can use them, and end up hurting future itemization or helping in power creep. Not to mention all those wasted database slots for items and the art assets involved.
