Why 3rd party driver in Driver Update Disk is not added into initramfs during installation of Red Hat Enterprse Linux 7

Issue

• We are using Driver Update Disk including 3rd party driver.
• We used inst.dd boot paramter, and it prompted the disk image correctly. And, it worked without any problem during installation.
• But, in the first boot after installation, it failed to boot since the 3rd party driver doesn't exist.