From “Paul Ryan’s Budget Plan: At Least It’s a Start,” James R. Stewart, NYT:
“The Ryan plan is clearly an opening move. … It’s easy to tweak the Ryan tax rates to generate more tax revenue while still lowering rates and broadening the tax base….
“In return, Republicans would get lower rates and curbs on entitlement spending. Democrats would get more tax revenue and preserve the essential elements and long-term solvency of Medicare and Social Security. The nation would get a fairer tax code, long-term deficit reduction, a secure credit rating, stronger economic growth and a social safety net.
“‘The overwhelming majority of Congress would agree to this on a secret ballot,’ [Jim] Cooper [D-TN] told me. ‘But they won‘t do it publicly. Each side considers it unilateral disarmament. The Democrats want to bash the Republicans on Medicare and the Republicans want to bash the Democrats on taxes.'” Emphasis added.
Winning re-election, remaining in power is more valuable to the individuals then running the country.