Questionable Questing

Because the fourth fucked up, or somesuch.
Or maybe that's just how Jinchuurikis are in that world.

You sacrifice a human being to serve as a body for a bijuu, while locking their power down (so it can be controlled) along with their memories (so THEY can be controlled)

Worst case scenario is them regaining their memories as a bijuu. Second worst case scenario is their personality as a bijuu waking up separately. Somewhere down the line is their memories from a previous jinchuuriki incarnation waking up.

Any of these and more can lead to the collapse of the whole house of cards.

More temporary stirrings and berserkings come with the territory of tapping their power.
A perfect jinchuuriki is one who remains attached to their human life and personality even after awakening to the full depth of their memories. This allows them to freely use their full power.

Knowing the truth about jinchuurikis...does not instantly make everything hopeless. It's still possible to keep everything buried while knowing, and a perfect jinchuuriki kinda.../has/ to know...but depending on the person and their personality/mental stability, it can easily cause a premature awakening and the escape of the bijuu.

In this context, the kyuubi would not have been partitioned. That just...wouldn't work. You'd get a permanently vegetative individual.

Pseudo jinchuurikis are a thing, but they /aren't/ that.
The theory behind them is that by placing a fragment of a bijuu's power within a normal person, it can grow and become imprinted with their ego, allowing them to develop powers akin to a weak jinchuuriki, or even go further.

'Going further' implies their own ego becoming imprinted onto the growing bijuu fragment in them, such that they can become a small-scale chakra beast and retain themself...essentially, evolving into a weak jinchuuriki with no memories to fear, and living on as a sub-tailed beast for at least a time after death.

The danger of a pseudo jinchuuriki is that the fragment of bijuu power in them is [hungry], [angry], and [hurt]. If they can't restrain and resist it, it'll devour them without becoming them, and then you've got the same being they could become...but a psycho killer at best, an unrestrained monster at worst.

I know a webcomic where the nine tails is reborn as Naruto. Do you want a link?

Sure. I mean, I've read some fics like that, but I mostly conceived of this as an 'ooh, that could be interesting', while trying to get a feel for early Naruto, so as to better write some possible snippets in it..whenever I actually can.

http://nekoni.deviantart.com/art/Naruto-Tensei-Chap-1-Page-1-54792164 Here you go.

Other thoughts: Jinchuurikis are sometimes made from the lesser chakra beasts, and have a somewhat better chance of becoming perfect (the undiluted instincts of a lesser chakra beast make up for the lack of memories) - but the benefits are just too small, when you're running up against people's fear of sacrificing their child and the knowledge that you can just kill a normal chakra beast and be done.

There might be ways other than pseudo-jinchuurikis that lesser chakra beasts arise, or it might be that rampages on the part of the bijuus (or out-of-control jinchuurikis) can naturally create pseudo-jinchuurikis, but there's a non negligible amount of lower beasts in the world - none of which have tails.