Except for frames, which I buy unassembled and put together...I build all my own equipment...Boxes, Bottom Boards, Ventilated inner covers, Migratory lids, Telescoping tops, and Nucs. I've also done a few single items...a Bee Vac, a Solar wax melter, and just finished an observation hive.
While I certainly don't mind saving a little coin, the most honest answer as to "why I do it", is simply because I enjoy it. While the savings compared to buying are significant, if I did not enjoy doing it and had to put a price on the time i devote to it...it wouldn't even be close to worth it.