"relatively low fps"
and you don't even give the fps you are getting.
What is relatively low?? to me that's like 60.. to other people its like 30... to you it may be 100...
details man..
I'm just going to attempt to answer this anyways..
First off.. SLI doesn't always work as intended. In games with poor or non-existent SLI support, you will be getting even less performance than you would a single 1080 Ti.
Having said that, pretty much any of those games should max out settings at 1440p and still get up there in fps barring a CPU bottleneck, which will still happen if the game is unoptimized. I play at 3440 x 1440 100Hz and there is exactly one game I cannot play on Ultra and get 100fps, and that is Far Cry 5 and I only have a single 1080 Ti, so really the only reason you are not hitting 165Hz is because of poor optimization or a driver issue.
So use some kind of monitoring software and check your PER-CORE cpu usage (overall tells you almost nothing) and your GPU usage when gaming.
If nothing is getting maxed out it is possibly a driver issue.
If the CPU is getting maxed out its poor optimization (because an 8700k should basically be able to run pretty much any game at a trot)
If only ONE GPU is being pushed I would disable SLI and see if it improves (SLI support would be poor or unavailable for that select game)