Available as in "you can expect it to be here by default". You're right though that there is no need for two editors. One of them is more friendly than the other (and even if I love ed, it is not it ^^), which might be why it is present.

Why do you call sam a "multiline" ed though? Because its command apply to the dot rather than linewise?
I love sam as well (and that is the reason why I use vis(1), which uses sam expressions and commands, wrapped in a vi-like interface), but I must admit that I don't know how it integrates within plan9, so I don't understand your comment about the OS level UI language. What do you mean?

Unix systems can totally provide GUI installations, and they do! see Ubuntu store for example which is nice.
Dpkg also has a graphical frontend, which is similar to Windows wizard.
I guess the CLI is better at letting people express what they want to the computer, rather than GUI. You have to learn how to "speak" your computer language, which is tedious, but in the end it is easier to bind your computer's will by explaining what you want to do, rather than miming it through the GUI