Because it undermines the integrity of literally every post on the site.
If the admins have the ability to modify posts and comments without any notice or indication, this opens the door to... - Creating false content via discrete edits that violate ToS, allowing for false flag to ban a user/thread/subreddit - Any comment or post used as evidence (ahem, e.g. Hillary Clinton's IT guy, mention of drugs or crimes, literally anything like that) now is called into question. Wouldn't surprise me if he now has to testify that posts were not altered if it is accepted as evidence, etc. - Literally anything else where a verified identity post and people assume it is what they (the person) said. AMAs are basically the thing that took Reddit mainstream, so that is kind of a big deal.