because
A both those games are heavily intensive, the game devs really pushed it hard to make teh PC versions something demanding, remember the old saying "can it run crysis?"
B unoptimized as heck 😉 pretty sure Metro is still not exactly the most optimized game, which on top of of its already demanding features really doesnt help
Vram is mainly a concern for resolution scaling, yes there is a benefit going from a certain amount to a higher amount, but at teh same time that "benefit curve" has a cutoff, and seems like 2gb is what did the trick at 1080 p, and most games barely even uses that much
thats why adding more Vram doesnt increase performance so much, unless you increase teh resolution, i.e. jump from 1080p to 1440p
you will see a significantly higher performance increase in say overclocking a 2gb card a couple of 100Mhz vs adding double the Vram on teh same card and having 4gb
its sorta the same with regular system ram, adding more will greatly benefit your system to a certain degree, like going from 2gb to 4gb,, but going from 4gb to 8gb the benefit lowers, and going to 16gb or 32gb, it completely vanishes, because the system simply never uses is like that
thats why you see people with 8gb or 16gb systems that perform completely the same compared to say an idiot like me with 32gb 😉
what you "do", ie the game/program/application need to be able to use what you have or its not being used, and games at 1080p simply rarely uses more than 2gb Vram (mostly about 1.5gb), and never 4gb or more 😉
ofc thats like saying all games only require a strong GPU and the CPu doesnt matter, there is always an exception to the rule, where some games can benefit a tad by more than 2gb vram, but still not much
and never to the point where the gtx970 would perform "poorly" on 1080p, then the error simply must either lie elsewhere on teh system, like a CPU bottleneck, or the game is unoptimized like crazy