As nice as it is to show appreciation for art by using it if you like it, there are the issues of commissioned art and what the artist wants when they make their art.
With commisioned pieces, those were (often) paid for by a specific person for their specific use, whether it's a custom portrait or an animation or a battle frame/UI or etc. This might be done because a creator likes a certain artist's work or because it's not something they can make themselves or because they want something unique to their project. (If there wasn't some level of artistic uniqueness and everything looked the same, then there would be less visual ways of differentiating projects, too!)
The other element is the artist's say - usually, as long as an artist is okay with their work being used publically and the person wanting to use it asks them first (or they specifically note that people can use it without asking), it's not a big deal. But, some artists have an aversion to their work being used as more than just art that they created and just want to make it and post it for visual pleasure and/or feedback and constructive criticism. Using art when the artist is not okay with it would be like going into an art gallery and making a copy of a painting on display that you liked so you could put it in the lobby of your business, despite it not being the original painting. (The other, other issue is one of intellectual property law.)
The same could be said for assembly, tools, maps, newly coded mechanics, etc. as well - while it's great when those things are made available and disappointing when they're not, it's ultimately up to the artists and creators to determine what they want made available and how they're made available.