Lol that’s done by volunteer community members as well. It literally is free to the ACM (but the people are paid to do it by their institutions as part of their jobs.)
We can argue/debate about what is the value-add (and precise workflow difference) of the IEEE editorial process vs. arxiv, but there is a difference.
Like the comment you responded to, I'm not defending the precise differential. I'm a past IEEE member (and author, and reviewer) and found their membership fees excessive.
In every ACM review committee I’ve participated in, it’s just volunteer members of the community emailing other other and using open source software to coordinate reviews.
Even taking into account overheads like desk space and pensions, that's a very large number of very well paid employees for basic secretarial work like soliciting reviewers