Ubuntu – Why is there no option to login as root

Because it is not recommended doing so unless you know exactly what you are doing. Root is a superuser, meaning it can do anything and everything - it is only meant for administrative tasks. Day-for-day tasks can bear a security risk that WILL be exploited if EVERYONE (or a large proportion of users) used root. Think Windows - 99% of malware and virus problems are from people using admin accounts for everyday tasks.

Let's look at an example:

What if someone breaks into your system and tells it to erase your disks? If you are running as a normal user, the only files you would (should) be able to remove are those that belong to you, and not anyone else. If, say, you had other people using the computer, their files will not be touched at all, no matter how hard you try. That means you will not be able to change any system files, and therefore your system will remain rock-solid and unaffected/uninfected.

If you run as root, however, you can wipe the hard drive entirely, probably do some damage to the hardware itself, and generally render the system unusable. Correction: anyone who can get to run a single command as root on your computer (some malicious website or an email attachment) can do that.

Take a look this nice article that might explain it better than I did.

Here's another one.

You can always use sudo command or gksu command to run the command as the superuser, both of those work with graphical and command-line utilities. sudo is usually used for CLI, though, and gksu is the graphical password promt that does the same thing.

To an extent, they let you become root temporarily - much, much better than running as root constantly and risking instability.