In part because it means that a property you take for granted in email (namely, that you can reference it later without it having changed) is violated. (This would permit all sorts of things involving documents being retroactively edited to whatever is convenient for the party sending the email.)

In part because you could use this to implement detection of an email having been "read" when the remote asset is fetched.

(These are just why I find those sorts of emails irritating and sometimes creepy.)