Yep, this is one of the most-often asked questions. As I understand it, from the original TNG story bible, the idea was that the Galaxy-class Enterprise was so advanced it didn't need a chief engineer. Obviously, they thought better of that, and thus the parade of at least four chief engineers and one assistant chief engineer, as I recall, from McDougal, Singh, Argyle, Logan, and Lynch.
Alrik's answer is pretty good, too, so if you want to accept that and make it part of your own personal off-screen canon, why not? I like to think myself that since Enterprise is Starfleet's flagship and one of the first Galaxy-class starships, all the engineers we saw were on temporary duty so they could learn as much of how the new ship's systems operate so they could then take their practical knowledge to other starships, not just the Galaxy-class.
Red Ranger