Try to monitor usage of ram during gameplay in taskmanager or some performance monitor.
Also you can have 16gb memory but OS limits usage for single process/program running. Before I think there was hard limit of 2/3gb max, but I think these days process can ask OS to give it more and bypass that limit. Maybe for some reason is game asking for more memory but OS can't/don't want to give it to it. On other hand even most demanding games should not use so much memory so something is probably wrong.
Make sure you got all latest updates for OS, gpu and also game itself to get possible fixes for errors like memory leaks. If it happens only on one of those games it may be error inside of game.