Imagine if the reason was not primarily about oil or money, but about geo-political shift. Iraq and Afghanistan are important strategic footholds in the Middle East. Whether or not there is a withdrawl of US forces in the next few years, strategic presence in the form of permanent military bases will be established indefinitely (see the end-game of WW2 for precidents). One can't be myopic about these things. This backs up an essentially neo-con agenda for expansion of US/European influence in the Middle East and beyond which has been ongoing for years. Ultimately the goals will include control over resources and politics in the region.
The point being that the US had been prior to 9/11 an increasingly isolationist country in terms of foreign policy. It had got out of the habit of acting independently overseas, due to lack of support for foreign intervention at home. Vietnam was the death blow to broad-based support for US "best intentioned" military intervention, plus the scandals which occurred when it got involved through covert missions involving the CIA and contra-funding. Even NATO-led military intervention was losing support at home.
The military was beginning to look like a massively-overfunded white elephant, with no obvious purpose either as a nation-building machine - standing up for democracy and freedom - nor as a defensive neccesity (who was attacking the US?). Budgets were being reviewed, cuts were to be made (even by Rumsfeld, who was shocked by the mad weapons contracts that existed, which swallowed money with little or no result). The arms industry gravy train was drying-up. The US military's influence was diminishing.
In terms of neocon geo-political agenda and both military and arms industry interests, 9/11 certainly came at just the right time. Before then it would have been very hard to gain support from the average US citizen for an interventionalist foreign policy or for more capital spending on defence.