Altonji, Bharadwaj, and Lange do not know.

They say:

The anemic response of skill investment to skill premium growth | vox - Research-based policy analysis and commentary from leading economists: The earnings premium for skilled labour has increased dramatically in recent decades. Yet... Americans are not acquiring significantly greater skills in response to this change.... Since 1980, the demand for skilled labour has risen faster than the supply of skills, fuelling a steady increase in the earnings premia found for measures of skills such as schooling or cognitive test scores. The rapid rise in the skill premium represents a substantial increase in the economic incentive to acquire skills.... [B]etween 1980 and 2000 the internal rate of return for completing high school rather than dropping out after tenth grade has increased from approximately 40% to 55%.... How rapidly and how much young adults respond to this increase in the returns to skills and how this response varies across the population have important implications....

In Altonji, Bharadwaj, and Lange (2008), we... look at factors that influence skill acquisition, such as parental education and growing up in a two-parent family... make use of measures of the ease with which young adults transition from schooling into the labour market.... [O]verall the 1997 youth cohort is more skilled than the 1979 cohort... at the median... skill[s]... increased by about 6.5 percent. Is [that]... a behavioural response by youth to the widening skill premium?... [N]o.... [M]embers of the more recent cohort have significantly more educated parents than young people in 1979.... Holding parental education, race and gender, and family structure constant, the supply response to the increase in skill premia between cohorts was small: about 1% on average and about 1.5% at the median.... It seems that very large increases in skill premia are necessary to induce young workers to increase their investments in skills substantially.... This implies that, all else equal, the large degree of earnings inequality observed today is likely to persist far into the 21st century....

[T]he difference in the skills of the 1980 and 2004 youth cohorts is larger at the top of the skill distribution than at the bottom... due to the changing distribution of parental education.... [T]he changing distribution of skills in the population will exacerbate rather than counteract the trend towards increasing earnings disparities....

At this point we can only speculate as to why the response in skills to the increase in skill premia is so small... non-pecuniary costs of skill investments... liquidity constrained... myopic... other reasons... consistent with a number of studies (e.g. Kane (1994), Dynarski (2003)) that find that schooling decisions are quite sensitive to direct costs of schooling and tuition subsidies.... Cunha and Heckman (2007)... [perhaps] parental investment during early childhood shapes the potential to acquire additional skills later in life....

At this point, the question of why the supply response to the increase in the labour market returns to skill has been so small is an open one. In our opinion, it ranks among the most important empirical issues facing labour economists today.