"What?" is the question, indeed. But it seems relatively safe to assume that the "institutional investors" came into here not for extra 10-20% profits. I don't know what can satisfy their greed, but I believe it's somewhere in +5x-10x-20x numbers. Especially if they manage to corner BTC and to convince others about the "digital gold" story.
Besides, they can't dump their holdings right now. If Grayscale starts dumping their shitpile now, the price will quickly go back to $5K and all Grayscale's paper profits disappear. The guys need buyers, much moar big buyers!