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@gbj_tester
Well, strength is seldom an issue, once you've made a crank stiff enough it's nearly always strong enough (tubular Ti Moratis excepted). After that, fatigue is a problem, mostly in aluminium cranks, which is where forging wins not only because it inherently strengthens the material but because it forces the designer to avoid sharp re-entrants, which are simultaneously a bugger to forge and the enemy of smooth stress distribution.
Milling stuff out of billets isn't inherently wrong (well, economically it usually is as long as your volumes are worthwhile), but on a component with high cyclic stress, you need to design carefully and quite possibly apply some post-mill surface treatment to eliminate stress raisers.
Billets are indeed forged, but the direction of flow of the material when forging a billet isn't necessarily the right direction for the finished article; imagine making a wooden plank with the grain running across its width.