Regardless of tax policy having been reformed several times, tax revenue relative to GDP has steadily increased. The problem is that the money is spent on other things, not that it's not being collected.
The increase is due to social security revenue, which goes into a trust fund and can't be invested directly into research. If you leave out social security, the graphs on that site show the remaining federal revenue steadily decreasing relative to GDP.
http://www.usgovernmentrevenue.com/revenue_history