I dunno, data entry isn't mentally demanding in the same way as some jobs, but that doesn't mean it isn't demanding. You're expected to enter at or above a particular speed, and it can be boring which is a different kind of mentality demanding.

did data entry for 2 months during a break from uni. It fried my brain. I was bored to shit and came home with a headache each day. I was so glad when I was done digitising all documents and was allowed to do some light programming instead.

I used to be a security guard, which is similar levels of undemanding. You try watching a bunch of black and white cameras all night, you get to the point where you go, oh look, a moth in genuine excitement.