Matrix In-universe perspective

To keep the sheep asleep. Also to have a movie. Plotwise that whole franchise is built on the structural equivalent of tapioca pudding.

It doesn't keep the sheep asleep because they can be unplugged from the Matrix in the "real world" and Morpheus only seemed interested in Neo (aka "The Anomaly.") No red pill-blue pill thing is needed to do that. "To have a movie" is the out-of-universe perspective. So from an in-universe perspective, why bother?

The machines knew about "The Anomaly" (Neo) and recognized a pattern was repeating. Humans playing around inside the Matrix wasn't going to break that pattern or machine control.

If a full-scale in-universe revolution wasn't going to change anything then it must have been the result of infighting. Maybe the beings in charge of the uber-matrix the machines were stuck in had to program it that way to keep the machines from breaking out and wreaking havoc in the matrices above them.