That's for efficiency reasons: most streams have way more logic than just filling the array, which can hardly be represented in terms of "getting the next value", and loops are good for optimization by compiler, etc. In fact, the actual sine wave implementation looks exactly the way you describe: it handles a generating function to a helper class that just fills the array https://bitbucket.org/lisyarus/psemek/src/master/libs/audio/...
It's interesting (odd?) that the interface doesn't just expose a function to get the next value, similar to a lazy list in Haskell, which also goes well with the original statement in the post that a stream is basically a function. The 'sine_wave' stream for example is 90% filling the array and 10% actual stream logic, which I imagine you're duplicating in other stream implementations.