Why is it so hard for government to have the budget it had in 2007?

If government agencies and programs functioned with 19% to 20% of GDP in 2007, why is it so hard for them to function with that percentage in 2021, when GDP will be substantially higher and with many opportunities for reforms and increased efficiencies? And if GDP and employment grow more quickly, as they would if private investment increased as a result of lower government spending and debt, then that 19% to 20% share of GDP could provide much more in the way of public goods.

And this tidbit:

...the chart shows that the second Obama administration budget, submitted a week after the Ryan House budget, is substantially different from the first administration budget. It is highly unusual for an administration to decide to submit a second budget....