So I think the answer is often ‘well, they often can’t go and get 3x the salary’? At least pragmatically, compared to theoretically?
Also, the multiple is closer to 10x for strong faculty candidates if we're talking about teaching-focused roles. A lower-tier college/university (non phd granting) will offer 60K - 80K for a full-time faculty position. A PhD with 5-10 years of industry experience will make >500K if they are maximizing for income.