For one thing, it is hard to know how much companies have in fact lengthened the maturity of their debts because the interest-rate swap market allows them to swap those fixed bond payments into cheaper floating debt, a popular strategy in the investment-grade market.
In essence, duration is a much better measure of sensitivity to changes in interest rates than simple maturity, because it takes into account when investors get their money.
Investors gravitated toward longer maturity bonds in an effort to secure higher coupons since older investors often rely on interest payments to sustain their income.