You also need to consider the utility of Life-insurance. Where would the proceeds go, who would it benefit, if you died would the payment from the life-insurance policy cover your spouse/kids needs in the same manner as if you were alive? The volaltiy of your HC does not matter if your PV of future cashflows is not needed to satisify a future liability. I.e kids going to college, bequest to your local CFA Society etc.
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Higher: More stable future income (not necessarily room to grow) to be earned. Said differently PV of future cashflows will be higher. The volality of HC is low therefore the discount rate is also low. Future cash flows are known and therefore risk (risk being death) of not earning said cashflows is high. Life-insurance is needed to cover the risk of not achieving said cashflows to fund known liabilities.
Lower: PV of future cashflows is uncertain. Volatility of HC is higher, discount rate higher as a result. PV of future cashflows to hedge with life insurance is low due to the uncertain nature of cashflows. Therefore the demand for a life-insurance policy to cover the risk of not acheiving known cashflows is lower.
I think the key difference is being able to measure, with degree of certainity being the discount rate, the future cashflows from HC in order to justify the ‘hedge’ of a life-insurance policy.