Jailbreaking is done to 1. Unlock the bootloader so that unsigned system code (other OSes, iOS downgrades) may be run, 2. Enable sideloading of applications from sources outside of the iOS or other "app store", and 3. Gain UID 0 (root) access.
On Android, "rooting" is almost always #3 by itself. The vast majority of Android phones already have bootloaders that will happily load kernels and OSes not cryptographically signed by the device manufacturer. In the few cases they are "bootloader locked", unlocking can often be done simply by using an official utility from the device manufacturer. Just as well, the vast majority of Android devices have the capability to install sideloaded applications. "Enabling" this feature in these devices is as simple as checking a option in the system settings. In short, we distinguish "rooting" from "jailbreaking" because "jailbreaking" always encompasses all three whereas "rooting" rarely involves anything outside of #3.