>>17
Functional programming languages don't insist that. Common Lisp has its loop macro, Scheme has iterative "do" syntax, and in Haskell the way you'd define factorial in practice is

fac n = product [1..n]

although you can do it iteratively too, if you must:

fac n = result (for init next done)
        where init = (0,1)
              next   (i,m) = (i+1, m * (i+1))
              done   (i,_) = i==n
              result (_,m) = m
for i n d = until d n i

Iteration is available in all these languages. It's just not the best tool for the job as often as you'd think.