Quoting Anthony wikimail@inbox.org:
Now, if you let the oversighters actually edit the old revisions freely, they could do what they've done in the past even more effectively. They could change my edit into a spelling fix, my summary into "fix spelling", and then change the following person's edit into a reversion of the spelling fix and the insertion of the bathroom overflowing statement. But that'd be way way too much power and ability for abuse. Not to mention unfair to the poor person making the subsequent edit.
And possibly leading to massive violations of the GFDL. Retroactive modifications to the history of this sort are just a really bad idea.