An installation of Ubuntu requires a minimum password length of 8 characters. (This was changed a few years ago, but I think they kept the installer with a shorter password length to make it easier to setup a user during installation).
Nevertheless, it is very easy to change the default password length...
With superuser or root privileges, you need to edit a file called /etc/pam.d/common-password
.
GUI Method
In the Unity dash, search for and launch "Terminal." In the terminal, type sudo gedit /etc/pam.d/common-password
. Enter your password, and a text editor will appear. Find the line that says "pam_unix.so obscure sha512", add a single space at the end of this line, and type "min=6" (without the quotes). Then save your file and exit the text editor. (This will allow you to use passwords that are 6 characters long. Of course, you could use a different password length number for "min=", if you like).
Simple Command Line Method
To make this easy, you can do all of this automatically by typing the following two lines into your terminal. In the Unity dash, search for and launch "Terminal." In the terminal, type (and enter your password when prompted):
sudo cp -v /etc/pam.d/common-password /etc/pam.d/common-password.original
sudo sed -i "s|pam_unix.so obscure sha512|pam_unix.so min=6 sha512|g" /etc/pam.d/common-password
(Note that the first line above simply makes a copy of your original common-password file, so you can revert to it if you need to).
A bit off topic, but regarding your other point about convenience, I find there are many, many conveniences with Linux, but there are always some things that do no come pre-configured "your way" out-of-the-box. For example, someone running a business with secure data may appreciate the longer password length restriction.
I also find that if something is not convenient for you out-of-the-box, then it can be easily changed to your liking. And the best part is, you don't always have to know what to do, because there lots of people out there willing to help!