AnymYo: Because RDR2 is using an improved euphoria configuration file from GTA5.
The common way to implement Euphoria physics to games was that a dev from NaturalMotion went to the studio and implemented it. That was the way, NaturalMotion distributed the euphoria engine. They did it this way with Star Wars TFU 1 and 2, GTA4, MaxPayne 3 and also Red Dead Redemption. After Zynga bought NaturalMotion, they stopped the distribution of the euphoria engine. R* was allowed to use it anyway because it is a core part of the RAGE engine. Before the release of GTA5, a Rockstar Games employee gave an interview and said that they will further on wont take the focus on "features which costs more money and work time and players wont notice at the end". What he meant was a detailed configuration of the euphoria engine and the gore as well. These two things were badly missed in GTA5 when you compare it with GTA4.
The funny thing is that the engine is basicly the same. So far, nearly all euphoria parameters from RDR1 are working with GTA5 and RDR2. Rockstar forgot some files in the GTA5 and RDR1 archive with all default parameters to configure the engine. I did a mod called "Cineastic Euphoria Ragdoll Rework" for RDR2 where these parameter are used. Shooting reactions now has all that classic euphoria stuff back (balancing, seld prevention, wound grabbing and so on). For me as a non game dev, it really wasnt that hard to implement this stuff. Rockstar or a NaturalMotion dev has all the documentation and could have done this in not time.
So maybe the reason why they decided to not make the physics that great and authentic like they did in RDR1 has something to do with a licence issue (with natural motiuon and the terms of self usage of the engine).
Now that TakeTwo bought Zynga (Zynga owns NaturalMotion -> the inventors of the euphoria engine) the possible licence issue should be solved. At least now, Rockstar Games should have full access to the animation tech. Lets hope that GTA6 will have the old authentic standards from GTA4, RDR1 and MP3 related to ragdolls. The best case would be if R* would improve the existing ragdoll configurations of GTA5 and RDR2 and deliver it with a patch but this will remain as a dream.