Remember that shinsoo gets thicker on every floor as the regulars keep moving higher and higher up the tower, In the first chapters when Hatsu is fighting with another swordman the guy says something like "These swords are too wide, let's use them while we can; as shinsoo get's thicker on higher floors we might don't have another chance". I believe this is also one of the reasons compression license exists, so that big guys like Rak don't experience excesive friction with shinsoo on higher floors.
So concluding, considering body armor's weight and volume it wouldn't make much sense to have it since it would also restrict their movements during battle; besides we know they can use shinsoo to reinforce their bodies and if they have a team (or are good at using lighthouses) they can create barriers, or use techniques like Novick's to make shinsoo force fields.
I've considered the entire setting to reach the conclusion I have, 4214.
The technology has been constantly displayed to us.
It is stupid of them to not take extra lengths to protect themselves.
It's like wizards not wearing armor in settings where armor would by no means restrict them from practicing their arts.
Not to mention that there are things like Archimedes or the Suspendiship Quant was flying around in his chapter, meaning that there must be some degree of metallurgical engineering in the Tower.
Not to mention that they've got a mineral whose main function is to float, which they've used to build things from weaponry (Ryun's staff comes to mind, aka a 2nd Floor Regular's at the time.) to super-advanced cube shaped computers, meaning that they have created alloys out of the stuff, enough of it to warrant commercial usage of it.
So weight stops being an issue too.
And they've also created machines that can manipulate Shinsoo to a certain degree, machines that can even raise somebody's Shinsoo resistance, so I'm sure as hell certain that there's a machine out there that can strengthen you too with Shinsoo.
Yet you tell me that not one jackass thought of "Hey, how about I put one of these pretty things into my shirt so that I won't be torn as easily as wet toilet paper when a guy with a sword/needle/hook/whatever-the-fuck-he'll-kill-me-with comes by and isn't exactly chanted by my sense of humour. I might not survive, but at least he'll have a bigger bill to pay when he'll have to send his tools for maintenance.".
People err, barriers and force fields created by humans(or otherwise) are unreliable.
A piece of metal would protect me more consistently, constantly and reliably than one of Novick's barriers ever would.
A piece of metal with the above precautions? Perfect. Sure, when you get it both with a Shinsoo strengthening mechanism and to be light because of a suspendium alloy, it'd be expensive, but when the alternative of not having is being DEAD, I'm sure I can live without sweet and sour pork for a few weeks.
They are being suicidally stupid, end of story.
I'm sure that the Royal Guard are regarded as such a threat because they're the only dudes bothering to wear a helmet around here.
Puts things into perspective on the supposed death toll of your everyday Regular.
Wangnan deserves everything that is coming to him in the Hell Train.