It seems like the plain strings are just ignored. Manufacturers brag about their wound strings, make many different varieties. Yet, there's so little info on the steel strings other than they're plain steel in order to be strong enough. Is there anything that can be done to make them more comfortable to play? Polishing? Different alloys? Coatings? Treatments?
What's comfortable is subjective. For example, skilled and experienced players with strong hands may prefer thicker, stiffer strings that provide a louder sound and a more precise pitch, while others (including most beginners) will find it easier to deal with thinner (and usually twangier) strings.
On the objective side, all strings are definitely not the same. Just like you said, there are significant differences in materials and processes. Even when it comes to plain, uncoated steel, there are countless different ways to make it, leading to different sound and feel.
Of all instruments, I think, these differences are most in evidence in the sitar. Many sitar strings are thinner than any guitar string, and the thickest almost as a thick guitar string. And they are all plain, not wound, and made of different materials: harmonic steel for the thin ones (which come in long coils which you buy by weight and cut yourself, the best quality of which interestingly is made in Germany) and a variety of materials for the others, including other kinds of steel, copper and bronze.
Bottom line, different materials, alloys, coatings, and manufacturing processes definitely lead to strings that feel and sound different. Some of their qualities may be to some extent objectively measurable (e.g. resistance to oxidation/rust, tension, sound volume) but many other qualities, including feel, playability, timbre, etc. will always be subjective.