Why are sent emails not deleted when I delete a conversaion in Apple Mail?
These are two totally different entities.
The confusion comes in from a misunderstanding of how messaging works.  It' helps to understand the concepts:

Your Inbox are what you received via your incoming server
&quot;Conversations&quot; is a collation of related messages you've received by your email client.  There's no conversation the server is aware of.
Sent Messages are messages that were transmitted via your outgoing server

What's important to know is that as far as your server is concerned, there's no such thing as a &quot;conversation.&quot;  There are just messages in folder (Inbox, Outbox, Sent, Junk, etc.).
&quot;Deleting the conversation&quot; is nothing more than a feature that was implemented to help organize your Inbox.