Probably because they are not finished making them. Will probably be fleshed out with the Mistlands update.

I freely admit to not knowing a lot about game asset design/creation, but wouldn't it be a fairly simple matter to have those angled versions of what we already have in horizontal/vertical format?

Every piece needs to be tested in a variety of conditions I expect, they would not want to introduce new bugs. However, it can also be by design and use the corner bracket design instead.