WantumBigger , I think the first mistake you are making is thinking that all drawings are equal.
Each drawing is different in some manner and, also, people who look at the same drawing will see them differently. Therefore, I may find the figure very appealing on the drawing of a beautiful woman in a bikini, yet wish she was nude. Someone else might wish she had larger breasts. It's all a matter of seeing something that looks good to us, yet thinking there is something about it that could be improved. Actually no different then someone's reason to morph a photo.
Your second mistake is assuming that the original artist is willing to make the change someone else desires (like drawing the breasts bigger).
You make the point that there already are drawings of women with "MONSTROUS tits". I, for example, happen to like certain artists that don't draw "MONSTROUS tits", but I like how
they draw the woman's figure. ..or I simply prefer that artist's style/method. Yet, if I want to make a change to those drawings, I can't ask the original artist to change them unless I'm willing to find and pay that artist whatever commission he/she may charge. Realistically, it's nearly impossible with drawings from Japan and only a little easier in the US, if the artist is even willing.
The motivation to morph a drawing is really very similar to the reason why people want to morph a photo. For example, why don't you tell
GALVATRON that he shouldn't waste his time morphing Jennifer Love Hewitt when he could be looking at Lisa Lipps photos instead.
I dare you!