I did actually already explain that one (spiritual beings regressing to younger forms as they lose power) but here's the full version from a while back when I got curious about this specific trope: https://docs.google.com/document/d/18xJbetvBNam51ZJaj7jdaMppRXls6ilrK-IoOQ3Czt0/

And to be clear, my goal has never been to justify the inappropriate sexualization of minors. Rather, my goal is to share the context for why these things developed as they have, and provide guidance on distinguishing between actual degeneracy and cultural misunderstandings. There is certainly plenty of anime with highly degenerate content, but there is also a lot of anime with content that appears to be a degenerate sexualization of a minor to a Western audience while a Japanese audience does not perceive it that way at all (rather than seeing it that way and allowing it)

Here's an excerpt, the summary conclusion:

The origin of the trope wherein long-lived, mature spirits revert to a childlike appearance when weakened is a convergence of religious history, folklore, and modern media mechanics. It is not a singular invention but a sedimentation of:

Theological Iconography: The Gohō dōji (servant child) and Wakamiya (new spirit) established the "divine boy" as a vessel of servitude and potentiality.

Folklore Dynamics: The Deceptive Disguise of Shuten-dōji and the Vital Fluid mechanics of the Kappa established that power can be masked or drained.

Modern Codification: Kishin Dōji Zenki systematized these elements into a "sealed form" mechanic, while Bleach and YuYu Hakusho added pseudo-scientific layers of "spiritual density" and "energy storage."

Visual Culture: The SD/Chibi art style provided a commercially viable visual language for this "diminished state," transforming the terrifying into the toy-ready.

Ultimately, the trope persists because it resonates with the fundamental Japanese spiritual worldview: divinity is not static or linear. It is fluid, capable of expansion and contraction, and intimately tied to the cycle of renewal. A weakened god does not end; they simply start over, small and full of potential, waiting for the prayer, the seal-breaking chant, or the return of faith that will allow them to grow once more.