Because it worksand has a good safety margin ...
I may be way off here, but I get the feeling that most Molle's we see are an over exaggerated modern take on the design and that in reality there was a blend of designs where the Molle is just the extreme end of the spectrum and the Meare Heath the other.
i think narrowing the tips is a natural evolution, wide tips just give you shoulders to act a nocks.... if you bind the tip with sinew then you can chop off the shoulders and go narrow.
I think we place too much stock on individual archaeological finds. That's what is so great about the Mary Rose Bows there are enough to be statistically significant.
I think the Molle is a slightly daft design... ok for accomplished bowyers but a nightmare for the newbie.
I've seen plenty where the levers were heavier than a bending tip would have been, and all they have achieved is to overload/overstress the inner limbs and add weight to the outer.
Del