> > The way architects think, it's considered more convincing to use materials honestly than to dress them up or conceal them

> That's not the way that architects think, it's a common current design fashion (and what we are discussing is interior design, not really architecture.)

It is practically the definition of being an architect to be concerned with truth to materials. No architect wants to be accused of designing paper-thin stage sets, dishonest buildings, things that don't actually work. It's important to distinguish (because this is a distinction that's very important in architectural culture) between the essential structure/function of a building, and features that are merely styling/cladding. The latter are to be avoided. This goes back to the beginning of architecture: a good building is decorated construction, not constructed decoration. Even those architects among the postmodernists who are worth seriously considering played by these rules. (There might be a few perverse PoMo exceptions)

An interior designer can't specify the details of exposed services, so this isn't really part of interior design (and anyway the idea of having exposed services comes from architecture) any more than a model holding an iPhone on the catwalk is part of fashion design.

> > So painting or otherwise decorating acoustic tiles is not an attractive option.

> It's absolutely an attractive option if naked elements don't support the ambience the occupant of the space wants. Which is why, despite the current popularity of the bare-services look, it is far from universal even in current (re)designs of spaces.

I meant attractive to architects.

> > The exposed ductwork thing is about not concealing (potentially messy) services.

> Yes, and that design trend is fairly recent, especially in it's application to dining establishments.

The specific "exposed services" trend is hot right now, but the idea of honest construction, truth to materials, form follows function, etc., really dates to the late 19th C. William Morris saw the shoddiness of mass-produced items (e.g. furniture) and called for a return to honest, simple, direct construction. Adolf Loos wrote about the redundancy of ornament, and the falseness of cladding (specifically, of making a material appear to be something it was not). The modern movement came out of an idea that that the customary, superficial, imitative forms of elaborate 19th C architecture could be replaced by a natural geometric simplicity. As Louis Sullivan said in 1892:

‘I should say that it would be greatly for the aesthetic good if we should refrain entirely from the use of ornament for a period of years, in order that our thoughts might concentrate acutely upon the production of building well formed and comely in the nude. We should thus perforce eschew many undesirable things, and learn by contrast how effective it is to think in a natural, vigorous and wholesome way. This step taken, we might safely inquire to what extent, a decorative application of ornament would enhance the beauty of our structures-what new charm it would give them.’

Exposed services, which have been a thing since the 50s, coming after the intentional rawness of brutalism (for example, see: https://en.wikipedia.org/wiki/Bowellism), are just an iteration of this impulse. For architects, carefully assembled "naked" services express and celebrate this natural honesty, functionality and directness. It's arguably an unjustified extra expense to get the services "right" (neat and tidy), but on the other hand, there's the long-standing traditional Medieval idea that even things that are out of public view will be seen by God, so they should be properly attended to anyway. This principle is a matter of thoroughness, probity and ethics.

> > It's not just a fashion,

> Yes, it is.

If I'd said "it's not a fashion", I'd have been asking to be corrected. But I said "it's not just a fashion", which is a very generous and weak constraint on what is going on–it's certainly visible right now as a fashion, but I'm claiming there's more to it than that, a history, a theoretical hinterland.

> > it's a kind of moral imperative,

> That's not as much of a difference as you seem to think: morality is in a very real sense just behavioral aesthetics; the broad outlines of morality may be fairly slow preferences to change, but you can find fairly slow to change elements in most domains of aesthetic. The attractiveness of the raw industrial look in interior design, however, is not among them, no matter how one might dress it up in language of morality.

I'm not going to claim that I can reduce the philosophy of morality to one sentence, as you have. It's definitely the case that ethics and aesthetics both concern judgements of value. (Historically they would both have been considered under the heading of axiology.)
Subsuming morality under aesthetics makes it something personal and sort of implies the transcendence of the judging subject, because only subjects have aesthetic experience. It is IMO very iffy, close to an https://en.wikipedia.org/wiki/Aestheticization_of_politics.

For architects, truth to materials is a "moral" imperative because it's thought of in terms of honesty, or a judgement of what is good rather than what is tasteful. I'm not making any broader claim that morality is unchanging. But this is a very long-running architectural design value, which just happens to be manifesting itself in certain conspicuous trendy ways at the moment. It's not a matter of "dressing up [the conversation] in the language of morality"; if you believe architecture is more than just eye candy, you can't avoid considering morality.

So we can agree that exposed services is often just a decision made on superficial aesthetic grounds. But IMO the reason why they are considered appealing has a lot to do with very deep-seated ideas about ethics and right conduct, and as you say yourself, those ideas change slowly.