Why does my Mom keep her eyes closed when she is awake in last stages of dementia?

This could be this early signs that the body is ready to shut down . Watch for molten of skin ( discoloration of skin) normally around heels of feet elbows and hands . Ears will fold in . That is how some people react to letting the body takes its course.

Early signs of body shut down it's trying to relax

In later dementia there can be visual problems that make the world even more confusing, I think that they are closing their eyes to block that out.

https://dailycaring.com/dementia-and-eyesight-an-expert-explains-common-changes-and-behaviors-video/

Probably too much visual stimulation to process at this point. Sorry.

My Mom did the same thing in the last 2 weeks of her life.

Every so often she would close her eyes. The weird thing was, she was lucid when it happened.