[runeks]

> Instead, it can be attacked simply by purchasing a lot of compute.

Firstly, no one is offering for sale enough compute to attack e.g. Bitcoin.

Secondly, and more importantly, this would be a temporary attack, during which the blockchain in question is rendered unusable. Once an honest majority of hashing power is reestablished, everything is back to normal. But a compromise of a majority stake is a permanent compromise of a chain. No honest actor can conjure up more stake to gain an advantage over the attacker who now sits on a majority.