Why do we need to reboot after the system is installed?

Whether we are installing from the live or the alternate installer, we have a kernel in memory.

Would it be possible to copy the contents of the ramdisks onto the hard drive, do other clean up tasks and then pivot_root into the new system? If running from the alternate cd, then X would be started...

There are occasions where a user can run the live cd, install some applications and customize the desktop only to have to repeat these steps after the installation has taken place. This would avoid that; the user would not have to repeat the same steps again.