Well, what would you do instead? It's an interpreted language. When you type "foofunction" at a prompt, you don't want it to wait around in case you define that to have meaning later.

You probably want the interpreter to be smart: "Am I loading a script from a file, or am I receiving instructions interactively on the command-line?" But now there's two modes of execution, and code in a bash script won't work if you type it into the prompt yourself. That's a bit uncomfortable.