I've seen some of the plans on the web.
Not all of them are traditional. Some are Burning Man hacks. Others are more conventional.
Some are heavier than most of the readymade commercial racks. Others are only slightly heavier, or comparable in weight.
Some are made by cutting and splicing old bike frames. Stays and dropouts from old racing bikes have been used.
Third-world designs are possible.
Some people enjoy doing these sorts of things, for one reason or another, in one way or another. Others want or need to save the money -- racks can be expensive.
If anyone has any ideas or links, please feel free to post them.
The recumbent world is full of things like "one-cent bents" that are made by modifying and splicing old bike frames -- there are all sorts of possibilities. Racks haven't gotten quite the same attention, but there is probably quite a bit of room for these kinds of variations and inventive creations.