Suspension of disbelief has nothing to do with it, for me. Not consciously, at least. I'm not really concerned about "looks like it could happen", I'm concerned about "looks good" or "fits with the rest of it".
Smooth liquid metal faces wouldn't look good, especially in conjunction with everything else about their bodies that's panel based. I'm not looking for that either. It might even be arguable that the current attempt at making the faces smooth (and/or easier to animate, idk) is what's led to this choice that doesn't work for me.
There's just something much less effective about, say, Optimus Prime's unmasked face (then and now) vs. those on Bee solo Clifffjumper or ROTB Wheeljack. The older faces were boxier and busier, but that works better than later takes that now really look like a face wrapped in foil. It's been a poorer execution that doesn't even really have any positive tradeoffs like better emoting or anything.