Because for you these are two distinct phases. But how many times you use a for loop for launching the same command multiple times? How many time you pass the o

Because for you these are two distinct phases. But how many times you use a for loop for launching the same command multiple times? How many time you pass the output of the command as a parameter to another with $(command)? Or you use variable to save things? Beside very basic usage, a shell is really a REPL for a scripting language. You can type commands but also you can do most advanced things (and that is what makes shell powerful). For example I maybe have to convert all the video files in a directory, and then upload them to a server, I can easily do that with a for loop in the shell, without creating a script. And since you use both the shell to type commands and to write effectively scripts as in a REPL, that means that you have to learn another completely different programming language, the fish language (of course I assume that you already know POSIX scripting), and not only that, it's probably the case that only on your computer you have fish but it's not installed on every other system, so when you are using another computer (a server, a computer of a coworker that you are helping, etc) you will make mistakes because you confuse the fish syntax with the one of POSIX shell. Yes, it's true that bash extends POSIX, and zsh too and it's not even compatible 100% with bash, but the differences are in things that it's rare that you will use in a basic script, and all these shells can understand POSIX syntax. A for loop it's the same in sh, bash and zsh (there is a shorter syntax in zsh, but the base one is the same), while it's completely different in fish.