For some the beauty of a finished kit, is enough to satisfy the hobbyist's need. For some it is not. Through either a lack of funds for the high end kits, or for a skills challenge, people choose to build things entirely from scratch, using parts from other kits, plastic sheeting, sculpting putty and even rubbish.
I myself have decided to venture down this route. I doubt it will become a permanent thing, as it is very time consuming. I imagine the kit I decided to imitate, had I purchased it, would have taken about a tenth of the time to produce, and will most likely be a better finished product. However, I am content with my efforts, and considering I was considerably determined to get the details right, am happy with the final outcome.
The above picture shows an actual Hell Blade (a sci-fi jet essentially) produced by Forge World, where as the image on below shows my attempt to replicate it. Whilst not 100% accurate, I feel it is a very good representation.