Same reason why EOD personnel wear 40kg of bomb suit while dealing with a small IED but generally tend prefer a t-shirt when it's a 500kg GP bomb.
There's is a limit upto which you can protect yourself and after that it doesn't matter because you've already hit the line of diminishing returns as now you can't even move because of the weight of your protection.
In case you want a long answer then we can approach this question from a different pov.
There are two loads with a soldier [I'm using Western standard as that's what I've studied, for our's there's very little literature available]; the approach march load and the fighting load. In approach march load you've things like MRE, extra water, sleeping bag, clothes, spare batteries and additional ammunition for a weight of atleast 15kg. Then you've your fighting load; rifle, 7 magazine, NVG, radio, camelback, grenade...this too is approximately 15kg.
On paper you can wish for anything; protection from shrapnel, a 155mm shell hitting you at chest, GAU-8, a meteor...but as soon as you realise that this protection (helmet + vest) would be on top of the 30kg you're already laden with; your will suddenly starts to shake.
So you start to negotiate between weight and protection; ultimately arriving at the optimum balance of protection on chest, back, head and flanks against rifle rounds (Lvl III) for an additional weight of 6.7kg.
• this is the maximum you can comfortably carry. Replace these plates with higher rated steel ones and you'll start sweating like a pig.
• whatever new heebie jeebie armour you see in all the exhibitions with protection on neck, thigh, belly...is never used in the fields because they hamper movement. This is the reason seasoned operators even remove the flank plates.
• soft armour is good for policing or concealed carry but is pretty much useless in LSCO. You can't stop anything beyond pistol calibre, if it's rifle rated then it'd be too thick and even if it somehow manages to stop a round you're still screwed as 1.2kJ of kinetic energy concentrated on an area similar to a fist would turn your internals into a mash.
• even lower tier hard armour can't stop artillery shrapnel, let alone soft armour.
These are your typical shrapnel, made up of hardened steel instead of the soft lead and copper of a bullet. Travels at astonishing speed because they're propelled by RDX, not a gun. The resulting KE is quite significant and also focused on a way smaller area.
The only practical takeaway from your idea is that regular grunts shouldn't wear high-cut.
But then that's what they use; be it MICH or LWH all standard issue infantry helmets are full covering