Stocks are not always better than no stock
While a stock gives a firearm a number of advantages, it also comes with its drawbacks.
- Better recoil control - Added weight
- Reduces hand-shock - Limits the postures the weapon can be fired from
- Easier to aim - Can become cumbersome in urban combat or other tight spaces
While most infantrymen agree that a stock is better than no stock, SWAT teams and vehicle crews often find that the stock inhibits them more than it helps; so, they will often be fielded with stockless firearms like the tactical shotgun or SMG shown below.
So, if your infantry already has too much armor for a stock to make since, it is probably because they are close-combat shock troops, in which case, they will find many advantages in ditching the stock anyway.
With Power Armor, your hand "becomes the stock".
Most of the future-tech settings the OP brought up don't just give soldiers thick heavy armor. They give them Power Armor. This is important because in addition to added protection, power armor can fulfil all of the roles that a stock is meant to serve which would make a stockless firearm universally more versatile in thier hands than one with a stock.
Using the the same sort of technology that cameras use to compensate for wobbling free-hand shots, your power armor could auto-correct the aim of a soldier by synchronizing the armor with sensors on the firearm itself. Even with a stock, a human can't accurately aim more than a few hundred meters without extensive training, but AI assisted riffles exist today that allow an amateur to accurately hit targets a kilometer away. So, with a little bit of auto-correction, you can overcome any need for better recoil control or easy of aim because your computer assistance is doing that part for you. The soldier just needs to point the gun more-or-less at the target to be able to shot it.
This just leaves the hand-shock issue which again can be nullified by the fact that you are holding the firearm with a robotically augmented grip. The recoil will simple go through your gauntlets and into the frame of your power armor instead of into your hand.
Since the power armor is doing the job of a stock, you get the better flexibility of a stockless firearm with out any of the drawbacks.
Why not a shoulder or chest mounted weapon?
When you mount a firearm directly to the body, it limits your ability to fight around your terrain. You can't use a chest cannon from a deep trench or prone position, and any position you can use it from generally involves exposing a much larger portion of your body to shoot. Shoulder cannons are a bit better, but limited when fighting around a corner or through a murder hole. Also, both options are a major sang risk when moving through woods, ruins, or other kinds of tight terrain, because a gun in the hand can be rotated a lot more ways to unsnag it.
Cover is one of the most important tactical considerations in any firefight because it both obscures your presence allowing you to shoot before you get shot, and it also minimizes your exposed body area making you a more difficult target to hit. Also, it takes more time to "step-out" to shoot a mounted weapon vs just reaching or tilting around a corner which gives the enemy more time to react.
When you use a hand-held firearm, you can reposition it relative to your body based on your combat situation; so, no matter what situation you are facing, you can always lead with your gun without having to expose a significant part of your body.