And not much motive to develop it. Slaves were abundant and the practice was widely accepted in paganism. This skewed the economics to favor making use of them.
Even in a world where slave labor was cheaper, machines were obviously better. Fewer people could produce the same output no matter how you look at it.
Modern machines are clearly better than slave labor today. However 2000 years ago a lot of things were different.
Some water mills were cost effective though.
[0] https://acoup.blog/2022/08/26/collections-why-no-roman-indus...
Slavery in various forms was widely utilized across all of humanity until the last few centuries, and in some ways it still is utilized today. We just don't call it slavery anymore.
"Paganism" has nothing to do with it.
This was a radically different teaching from other world views at the time, paganism being the main one in question during Roman times.
Paganism may or may not have specifically promoted slavery, but it did not offer any teaching that there might be a fundamental problem with it.
Framing slavery as rooted in paganism isn't accurate, and more importantly isn't helpful in having a worthwhile conversation. All it does is paint Christianity as some moral paragon, which it is not.