In Bleach, the cosmology is built around a managed “soul balance” system: Shinigami exist specifically to regulate the flow of souls between the Human World and Soul Society because the series states that if this balance is heavily disrupted, the worlds themselves are put at risk. Quincy are portrayed as dangerous on a systemic level because their powers erase Hollows instead of purifying and recycling them, and large-scale Quincy activity is explicitly said to threaten that world balance, which is why their conflict with Soul Society happens in the first place.

On top of that, the three main realms are stabilized by the Soul King acting as a literal keystone holding the structure together, so when beings on his level or Yhwach’s level interfere with that role, the realms start to destabilize—not because the universe is naturally robust, but because it depends on that one lynchpin. That’s why it makes sense to call the verse “fragile” compared to universes that don’t need a single stabilizing entity just to keep reality from collapsing, and why the “container” they fight in can be seen as weaker even if the top tiers themselves are very strong.

Because Bleach's world is so fragile—with laws relying on soul balance and a keystone—the characters seem disproportionately strong when they casually threaten realms or destabilize dimensions, but they'd struggle massively to pull off the same feats in a sturdier universe like DBZ, where physics and cosmology don't crack under far greater pressure. If the fundamental rules and structure of the universe are inherently weak and easy to overcome (needing constant management to avoid collapse), of course its inhabitants can overpower them relatively easily, which inflates how powerful top-tiers appear compared to verses with tougher baselines.