Why Isn't SSA A Cabinet Level Department?

I thought I would compare employment at the Social Security Administration to that of departments in the President's cabinet and to other independent agencies that have more than 10,000 employees. This is based upon figures from the Office of Personnel Management.

• Department of Defense (civilian) 668,450
• Department of VA 239,689
• Department of Homeland Security 168,635
• Department of Treasury 106,623
• Department of Justice 105,827
• Department of Agriculture 105,488
• Department of Interior 73,126
• Social Security Administration 63,647
• Department of HHS 63,506
• Department of Transportation 53,861
• Department of Commerce 40,544
• NASA 18,457
• Department of Labor 15,339
• Department of Energy 14,950
• General Services Administration 12,170
• Department of State 10,208
• Department of HUD 9,825
• Department of Education 4,344
  

So why there has been never been a serious proposal for Social Security to become a cabinet level agency -- or at least not in a very long time? It cannot be because Social Security is not a big ticket item. Social Security involves more money than any agency other than the Department of Defense. It cannot be because what Social Security does is not important to the American people. About 50 million Americans rely upon Social Security for all of part of their income. Is it because we want to keep Social Security "out of politics?" The agency was part of a cabinet department for most of its existence without any sign of obvious harm. If anything, Social Security seems to have been more "political" since it became an independent agency. I do not recall anyone like Andrew Biggs working at Social Security before it became an independent agency. Also, I do not recall any President traveling around the country trying to privatize Social Security until after it became an independent agency.