Why doesn’t a higher life insurance premium always match the benefit increase?

Like so many other products in the US economy, life insurance pricing is not linear. Sometimes it pays to buy in bulk. There are many reasons for this situation. As stated previously, the pricing results from complex statistical analyses completed by insurance companies and their actuaries. Insurance companies are not giving a "bulk discount" out of generosity to their insured customers.

Insurance companies collect revenues in the form of insurance premiums, which they invest to earn interest. From these " revenue sources," they incur some consistent expenses, such as wages/salaries, overhead (utilities, rent, etc.), and marketing. These costs are relatively predictable and similar for any policy, whether it’s a $100,000 or $500,000 policy. Insurers then have an uncertain expense in the form of a payout when an insured member passes away. 

The variable cost is whether the death benefit will occur. So the pricing scenario described in this question reflects a lower probability that the individuals with higher death benefits will occur as soon as those with lower death benefits do or that the forecast of interest earned on the additional premiums will be statistically sufficient to compensate for the potentially larger death benefits.