STEM encompasses all of science/engineering teaching, but in the context of STEAM, STEM is "how to make a thing", and STEAM additionally addresses "why to make it, how it should look" etc. I think the "A" is also likely a rather restricted subset of Art, in this context.
The STEAM idea has been around for some time. I first heard about it from John Maeda like 7 years ago (https://www.youtube.com/watch?v=0yc-_Ei2Yto).
"STEM" by itself lacks an emergent, creative quality. There is very much a need for that in technical curricula.
I think many would argue otherwise, there is an inherent level of creativity with regards to all four disciplines, in particular Engineering, where novel solutions to real-world problems can be both practical and creative, although you might argue this extends into the other three as well.
Those strike me more as philosophical concerns than artistic ones.
Art isn't about explaining things, it's about expressing things. STEM is something along the lines of how the physical world works. "Why", almost by definition, is a question of philosophy. How it should look is a question of aesthetics.
So, in my opinion, maybe the synthesis is something like: Philosophy is "why", STEM are "how", and Art is expressing our answers to those questions and our feelings about them.