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What I have gathered after seeing threads like this pop up on Reddit periodically over about 11 years, and most of the responses here, is...
Fuck if there's a reason.
No, seriously. Pretty sure it's become standard and as Capitalism and Management do, they follow whatever bullshit tradition exists.
That said, I personally have always felt factory and retail work fuck with schedules so much because it maintains control and limits sociability outside of work and work groups. Thus increasing retention by artificially creating social nodes we feel a part of and/or do not want to be removed from. Likely created by the same people who cut pensions and created things like Mining Towns.
Without knowing anything about the situation, my guess is that if you run a static 1st, 2nd, 3rd shift style rotation, you can pay market rate for 1st and 2nd shift, but you have to pay a premium for 3rd shift due to it being a less desirable shift for the general public.
With rotating shifts you just hire people and make them work the graveyard shift for no additional money, and get to argue with those that have to work it as "look, no one wants to work 3rd shift. That's why we rotate it between everyone on. We're a team here. We all pull our own weight". Which is a bad faith argument, but they'll happily use it on you and frame you as the over demanding worker, instead of the underpaid worker.
This is it. If they're not technically third shift, they don't have to be paid a shift differential.
They can claim everyone is "normal thing, payroll-wise" and the fact that a full third (or 2/3) of the work they do is "outside of normal" can be classified as "as needed" and "occasional other shifts as required".
It allows them to be one thing on the books while being completely different in practice.
100% they fuck with schedules. Intentionally to stop you from being able to find or transition to a second job. Easier to pay bullshit wages if you have nothing to fall back on.