game of thrones - DragonBone usage in the GoT/aSoIaF universe

People do use bone for armour / decorative armour; see Rattleshirt, albeit he uses regular (human and animal) bones.

It would be a matter of workability and accessibility.

Dragonbone is highly sought after (especially by the Dothraki):

Dragonbone bows are greatly prized by the Dothraki, and small wonder. An archer so armed can outrange any wooden bow.
^{-A Song of Ice and Fire: A Game of Thrones, Chapter Thirteen (Tyrion II).}

And apparently Magister Illyrio is a dealer in Dragonbone:

Dany said nothing. Magister Illyrio was a dealer in spices, gemstones, dragonbone, and other, less savory things.
^{-A Song of Ice and Fire: A Game of Thrones, Chapter Three (Daenerys I).}

Bear in mind though, that the last Dragons (before Dany's) died in 153AC (over 100 years ago) and as such is a rare commodity, which is why you wouldn't see them around.

Targaryens were renown for their ownership of Dragons, but I couldn't see them turning their dead Dragons into armour / weapons. We see that most of the known Targaryen Dragon skulls were kept in the Throne Room (and most recently in the Cellars). So in a way they were honoured as prized possessions - not likely to be made into tools.