No "global" hoax or conspiracy is required for humanity to be stupid and wrong as it always is.
The assumption that the earth was spherical was first posited by someone that sat on their ass (while diddling their boy slaves, undoubtedly) and made it up over 2000 years ago. It shouldn't cause any shock that they were wrong (or at least easily/likely could be), and only minor shock at the fact that prior to nasa et al (if you believe everything you see on tv) that the assumption had never been validated by anyone, ever.
There is still a lot of (indoctrination towards) ancient greek worship about (and the pedophilia they so adored...). It is the foundation of fraternities and many other "traditions" and is generally rife within "academia". One can't help but wonder what else about the world might be different if they had spent less time fondling, admiring, and worshipping each others balls thousands of years ago (and things they imagined resembled them with no adequate reasons or reasoning)
Perhaps we were too busy venerating/idolizing instead of scrutinizing. One of the MANY flaws of the modernists is they stupidly think that there is no reason to study the past. The foundations of science are back there, and what egregious flaw (that we have grandfathered in, due to greek ball worship and other things) is inevitably there will only be found through rigorous scrutiny/critical evaluation.
The existence of a hoax, or not, is irrelevant to the shape of the earth, and to determining it with certainty. Discussion of conspiracy and hoax, although entertaining at times, is red herring and distraction from the real topic at hand.
Speaking of which, back to the OP's question.
IF there were a conspiracy to hide the true shape of their world from "average citizens/employees/peasants" it would likely be for the purposes of warfare and domination.
For example, we know now that the american continent was well known by the aristocracy in europe for centuries (if not millennia) before columbus, was likely the source of the copper from the bronze age, and was on many of their maps.
It was very important to them to lie to their slaves/employees/citizens/peasants to stop them from leaving to try and have a better lot in life (or at least the possibility for one).
In war, there are really only two things - topography and subterfuge, and bad maps tick both boxes in a major way.
What if there were somewhere better to go? Just hypothetically of course