^ Maybe the question to ask is not "Why do they make so many of them if they are so cheaply constructed?" but rather, "Do they make so many of them BECAUSE they are so cheaply constructed?" Maybe the lack of armor allows for them to be less expensive for mass production. They're essentially expendable cannon fodder. Which of course doesn't mean they're so cheap that the Captain doesn't get pissed when Kennex blows them away for no reason, but still, cheap enough to be ubiquitous.
Then there's the other factor you already mentioned. You don't want them to be nigh indestructible physically because even with their relatively fragile bodies if they rebel they can still take out dozens of cops with their superior strength, speed, stamina, agility, and situational awareness. So that fear might have caused their creators to leave some disadvantages.
Now, you'd think with the DRNs where they were trying to make them more human, they would eliminate that weakness, but what if it's an essential part of making them think in more human and humble terms? If they were virtually indestructible, they might come to think of themselves as superior beings to humans. But if you engineer in a fragility to their existence, they might try and preserve it and consider their life, and the lives of others, precious. Obviously that didn't work out with the XRN, though.
Then there's the other factor you already mentioned. You don't want them to be nigh indestructible physically because even with their relatively fragile bodies if they rebel they can still take out dozens of cops with their superior strength, speed, stamina, agility, and situational awareness. So that fear might have caused their creators to leave some disadvantages.
Now, you'd think with the DRNs where they were trying to make them more human, they would eliminate that weakness, but what if it's an essential part of making them think in more human and humble terms? If they were virtually indestructible, they might come to think of themselves as superior beings to humans. But if you engineer in a fragility to their existence, they might try and preserve it and consider their life, and the lives of others, precious. Obviously that didn't work out with the XRN, though.