They're *not* actually machines though.
Lore states that they're more or less re-animated corpses. That's not going into further detail regarding the Infestation and what not, but the all boils down to the insides of a frame being organic. Hell, you said it yourself.
Frames that are bodies that require the usual functions to survive. Bodies that require armor for protection. That's why frames are capable of bleeding.
The fact that they are bodies further support why those kinds of designs *can* make sense.
I also think you're just looking at a coincidence. There's a number of skins that get in even in recent times that reflect the fact that underneath the armor *are* bodies, and thus use clothing or accessories because it just looks more natural than just... bolting plates of steel all over a frame as if they were a tank.
An older but DE designed example would be the Excalibur Proto skin. Newer examples would be Mesa Prime, who I swear looks like she's wearing ass-less chaps.
Not to mention Limbo, who is quite literally wearing a suit; complete with cuffs on the wrists. Why would a frame need coat tails?
It's because it's just a cool looking style, and shouldn't be looked that far into. That's why having belts and S#&$ on a frame isn't that crazy.
There's a few lore-breaking skins such as the very, very recent Harrow deluxe skin where he's missing a huge chunk of his torso, but hey; atleast he seems to still have a heart in there. So I don't think DE's problems is down to your specific theory.