It would be very convenient to edit comments any time you want
Comments are not intended to be permanent, important or prominent. Allowing arbitrary editing would require keeping a edit history and making it available to people trying to sort out why the comments now seem to be confused.
Comments are intended to be a lightweight mechanism for requesting clarification and suggesting minor corrections. Yes, they are used for other purposes, but the team have been adamant that those are off label uses that have no official status.