> Why is it that in such cases, the government reaches for the CEO right away; but when Wells Fargo employees open millions of fraudulent accounts, it's not the CEO's fault?

Probably because a corporation like Wells Fargo will have extensive documentation of procedures and training that establishes sufficient evidence that there were efforts to constrain employee behavior to lawful acts in pursuit of the metrics which they ended up maximizing through unlawful acts that it will take a lot of digging, at a minimum, to have a viable case of executive criminal culpability.