There's a real conflict here between the expectations of individuals and the expectations of the group.
The individuals are usually told via user agreements that the content is theirs, and that they own their comments and data and are personally liable for the content of them. That is... if you libel someone, this is your problem.
And the group expects that once a collaborative work has been created (a conversation), that the work will stand in it's original form. And that implies that when others post a comment within the context of some greater conversation, that the context for their comment is preserved.
Therein is the conflict. If an individual deletes (physically removes) their data (we did tell them it was theirs), the collaborative work is affected and the context of everyone else's contributions is changed.
To resolve this nearly all fora work on the basis that you can nuke your account but not your comments. And if you do nuke comments, that you can only nuke a subset of them and that a marker will be left to indicate that a comment once existed.
This works about as well as you imagine, it pleases no-one fully and everyone just a little.
The alternative that is also taken by some fora is to have your contributions licensed in such a way that the operator of the site has a perpetual right to reproduce the content. Great... the group gets their value, but you've lost control of your content whilst still being liable for it.
And who is liable for a comment that now shows up with a [deleted] author? Likely as not, it's the site operator.
Everything on comments is just a shade of grey in this conflict zone.
When it comes to accounts. I fully agree with you. The account isn't a collaborative work, and it is solely your work.
The vast majority of fora allow account deletion by policy, but the software tends to not have caught up with the policy. Meaning you have to email the site owner and ask for deletion. On the sites I run (mostly vBulletin, some XenForo, a few Vanilla) I do deletions within a week or two of being notified. As the software doesn't support undeletion I tend to warn people and give a cooling-off period until a confirmation from the person is received to acknowledge it's an irreversible action.
I'm rambling... gist is: deletion of comments would affect the collaborative work that is the essence of a forum and should be managed with extreme caution by the site owner and attempt to leave things in context. Deletion of user profiles has no such impact and should be honoured, but unfortunately the buttons to achieve this tend not to be built into the underlying software.
And on a final note, I think systems that support aliases should be allow people to rename themselves too. But that's an even less-supported thing than profile deletion.
Forum software sucks, but it's not all the fault of software most of it comes from the social stuff surrounding a collaborative work.