The miraculous image rehabilitation of former Republican presidents

Why does the media despise a sitting Republican president and only ex post facto reinvent him as underappreciated?

A cynic would suggest it is not because of any fair analysis of comparative presidential achievement. Instead, the skit goes like this:

Once a Republican president loses an election or retires after two terms — and is followed by a liberal Democrat — his reputation hits bottom. But once a new Republican president enters office, the prior and now-powerless Republican ex-president is airbrushed into a model of statesmanship to contrast the ogre currently in the White House.

Republican presidencies are seen on a downward spiral of always becoming worse — by always redefining formerly despised presidents as at least better than their monstrous successors.