Because parsing +(1 2 3) requires lookahead. You want to be able to type X into ...

Because parsing +(1 2 3) requires lookahead. You want to be able to type X into a REPL and get back the value of X, but you can't do that if there is a possibility that an open paren after the X will change its semantics into a function call.

Also, what should be the result of:

    (defvar f ...)
    (defun f (x) ...)
    (length (list f (g x) f(g x)))

?