Why, because all gas operated systems a delayed blow back. What you are arguing whether you realize it or not is whether roller-block locking is better than another locking system.
I don't believe that it is. There are more moving parts and more wear points than other locking systems, frankly, from a engineering view-point it is a complex locking system, and complex is never the best design approach. In engineering the "KISS" principle is alive a well. Simple is always better. As a reference point, look at the AK-47 locking system.
Thus, this comes back to point about I made before about ammo and weapon maintenance.
But, as a counter-point to your argument assuming for a moment that HK engineers are smart guys and have done a lot of testing, why then have their new weapon systems gone to a different design? Further, if roller-block locking is best, why was there so many design changes done to the MP5 locking components. I'm not saying the roller-block locking is a bad system, all I saying is I would not call it the best, a good design sure, the best, no way, the best has been invented yet.
Frankly, I have not seen a single gas operated system that can compare to the reliability of the Mauser 98 bolt action design, but that is a horse of another color.
Now taking another tack on this subject, which you have not addressed yet, every gas system was designed with a vary specific cartridge in mind. Thus things like bullet shape, gliding metal (jacket), barrel friction, powder pressure curve, etc all factor into the the design and operation of gas systems. Changes in powder charge alone, not to mention using different powders with different pressures all impact the performance of the system. In my previous post I said all gas systems have issues with cartridge/ammo used. That statement is still true!
Regardless of the locking system, the bolt release is a function of the bullet passing the point in the barrel where gas is vented back to bolt carrier which causes the action to unlock. Actually, strictly from a engineering point of view the stoner design is better as the moving mass is inline with the barrel. There is torquing effect that interferes with follow-on shots on the HK-91/PTR-91 and the AK-47.
And while you may disagree, my combat experience tells me that empty mag bolt hold open impact is a serious design defect! You are entitled to disagree, I'm just expressing my opinion, which I claim is no more or no less valid then yours. As my battle rifle, I agree that the kind of home defense combat I might experience it probably is not a big issue. However, on a real battle-field, it would be an issue for me, and that includes places like Iraq and Afghanistan.
I hope I never see real combat again, and I am grateful to all of the men and women who do or have served. I honor their service!
In closing, as before ....
Buy and hold is my method of gun ownership!