I dunno, Lelouch has repeatedly demonstrated a pointed unwillingness to learn and change from mistakes if learning from his mistakes means actually having to stop doing something he wants to do/believes necessary. See also: "Rebellion is getting innocent people killed and hurting people who you care about" -> continuing rebelling in the exact same way; "Geass can lead to angst and loss of control and craziness" -> continues using Geass.
Zero's just another thing that Lelouch doesn't actually want to give up.
Re: , but...
Zero's just another thing that Lelouch doesn't actually want to give up.