I have a theory about the overboiling of veggies, based on the origin of the culinary cabbages in the form of seakale. Seakale is thick leaved and would be really impossible to eat without extensive cooking.

On the other hand, I have seen in passing and not referenced some idea based on the theory of humours, which made the removal of greenness and crunch absolutely essential. This from people who believed in bleeding patients.

Here's some of the logic.
Terence Scully, in his book The Art of Cookery in the Middle Ages, has this to say about the preparation of vegetables:

"Most vegetables, whether leaves or roots, were chopped, ground and cooked by boiling. The logic of this cooking method lay in the tendency of vegetables, being products of the earth, to be dry in their nature (the exceptions being vegetable marrows, melons, & chard, all three of which were held to be rather moist); the action of stewing vegetables lent them the moisture they lacked by their nature. Members of the onion family, however, being moist in the third, and even fourth - or most dangerous (even mortal) - degree, were usually fried, thus removing a little of their superfluous moisture."