Because a good MOC must first have a good design (something strong, optimised, etc).
And the fact that on EB I can post as much photo as I want (but I remain reasonnable ; all the photos on this page are not very big), is very useful for me. Because it enables me to explain many things. On the contrary, on FBTB I can not post as much photos as I want, and that's a big problem for me. For me, presenting a MOC is not giving a link to a gallery. It is much better with the photos in the thread, with comments on each photos.
I never buy sets for their parts. If I want parts, I go on Bricklink. So, I get only what I need.
Besides, my collection is not so big to do MOCs ( http://www.brickset....h/?OwnedBy=Anio ). All the UCS models are built. I don't do anythings with my Technic, because it is studless (MOCing with studless is not what I prefere...). Yesterday I bought the 10212. Like the other UCS, I will build it and put it on a shelf. So, I don't have any spare parts with the models I buy.
Moreover, I hates having many parts in my storage boxes. Because when I look at them, I think "Do something with these parts !! You can't let them like that !!". That's why I rebuilt my 10030 ASAP. More generally : the fewer useless part I have, the better it is.