I was going to leap in with a knee-jerk denial of this, but I think it’s probably substantially true. People really did thoroughly/over cook their food - not just veg - in fairly recent history.
I think some of it does derive from basically ignorant mythology about nutrition and digestion - but going back a little further, there was a distinct class division - you had the richer folks eating a fair bit of meat, and any vegetable accompaniments being subject to lengthy and elaborate preparation - to make them conspicuously fancy.
At the other end of the scale, you had poorer folks eating mostly vegetables, but to make the small amount of meat go further, they might cook it all together in potage, stew, or pudding - one of the advantages of these is that they can be left to cook unattended while other household tasks are taken care of.
Going back further still, it would have been the case that many vegetables needed extended cooking - just to make them palatable - early forms of cultivated vegetable (or wild ones gathered for sustenance*) would often have been coarse, fibrous, bitter or pungent and a long simmer in the pot would have rendered them pleasant. Old habits die hard.
*I mention this because it’s possible to go out and pick small quantities of delicious, tender and tasty wild vegetables (and this is something I do myself from time to time), but If you’re gathering wild vegetables in large quantities to feed a large family or a small community, you can’t be so discerning as to only pick the tender tips as you skip gaily along the hedgerow.
I expect there are other contributory factors