The issue with memory paging on the apps is about what it does for HDD access guarantees.
Worst case mechanical disc seek times are in the 30ms range (if your reading from say the outer edge and the page file is on the inner edge).
So you add a potential 60ms plus the read/write cost to your potential latency for any issued HDD access.
You already have that issue if the video buffer is streaming to HDD.
They could certainly do it on the App side but not without it having some impact on the games side.
Worst case mechanical disc seek times are in the 30ms range (if your reading from say the outer edge and the page file is on the inner edge).
So you add a potential 60ms plus the read/write cost to your potential latency for any issued HDD access.
You already have that issue if the video buffer is streaming to HDD.
They could certainly do it on the App side but not without it having some impact on the games side.