My own feelings are
1. It's a combination of casting design and balance job (which many for some reason think they'll "feel" if it's "bad") or a combination of those two and a cheap harmonic dampener.
2. Detonation. It's detonation that makes the caps walk and you can find that on a factory stock engine. You can't always hear it, but the block will always feel it.
The most I'll invest into a factory block is aluminum caps and main studs. Those two, with a properly balanced assembly, and a good dampener will withstand a lot of abuse at the 600+ hp range and doesn't break the bank to do. The caps absorb the shock loads rather than transfer it to the webbing and the studs keep them basically where they need to be. Aluminum rods and a belt drive are also great tools to help the block live.