Preinstalled iLife on my MacBook disappeared after factory reset

They did not disappear. You erased them when you did your factory reset. You will find it on the original discs that came with your computer or you can re-download them if they were purchased from the App Store. Or you may be able to restore them from your backups.

iLife is not a part of OS X and never has been.

Installing Bundled applications such as iLife (via Applications Install DVD)

1. To install the bundled applications, locate the Applications Install DVD that came with your computer.
2. Insert the Applications Install disc into the optical drive.
3. An Applications Install window appears. Double-click Install Bundled Software.
4. The Bundled Software Applications installer window appears. Click Continue.
5. A software license screen appears. Click Continue, read the terms, and click Agree if you accept the terms.
6. To install all the bundled applications, click Install.
   To select individual applications, click Customize. Then, click the disclosure triangle next to Bundled Applications so you can select individual applications. When ready, click Install.
   Note: Clicking Standard Install on the Customize screen installs all bundled applications from the disc.
7. When prompted, type your admin password to start the installation.

http://support.apple.com/kb/HT3415?viewlocale=en_US&locale=en_US

Mason M. wrote:

..erasing the hard disk using disk utilities and resintall the OS using the included CD

To do this you had to have used c or option key to boot from the disk

Now that you have OS X running on the machine from the internal drive, stick either disk in and look for the bundled application installer and run it.

Last resort is to use Pacifist from CharleSoft and extract the programs from the disks