Even though the population of the United States has grown, the taxes received by the federal government has dropped (as a percentage of GDP). Part of this is because of the massive tax cuts the Republicans gave to the rich and to corporations. But there is another reason also. Those same Republicans have been cutting the money budgeted to the IRS -- especially the money for audits and investigations. This makes it easier for people to hide income and avoid paying their fair share of taxes -- especially the rich.
These charts, from ProPublica, show what is happening. They have also written an article on this which is well worth reading.
