The problem with making a shell more like a "real" interpreter REPL is the purpose of a shell: Running arbitrary programs not known to the author of the shell, such that any unknown word in a shell script is probably the name of a program as opposed to a misspelled reserved word or variable name. Those programs having arbitrary command-line arguments and having to operate on files with arbitrary names is another complexity.
It's interesting to see how so many things grow into each other. A shell is what you say, but there are many efforts to change that. Bash has its magic autocomplete which knows about options of many, many programs. Powershell has an object-model with knowledge of many of its commands.
