I hear it a lot but I just don't understand, why would the government want to hide the truth from us?
There isn't a single rational explanation that holds up to scrutiny.
I want to eventually address the conspiracy claims, because that's what this comes back to, but the bottom line is this...
Let's assume the following:
1) The earth is a flat, motionless plane beneath a dome.
2) There is a massive, elaborate, global (haha) conspiracy to hide the reality of the natural world from the public at large.
3) The primary deception in this conspiracy is that we are 6-foot tall, upright, bipedal, binocular space monkeys living and dying on a massive, round, oblate spheroid spinning on its axis and revolving around the sun at hundreds of thousands of kilometers per hour in an infinite void we call "space." There are of course more details, but that's the primary deception.
If we assume that all of those things are true, and we ignore investigating conspiracy claims any further (because we believe it), the first problem arises when we attempt to reconcile the flat earth model with reality, i.e. the body of observations and experiments we can gather in reality CONTRADICT the first assumption. Those observations are NOT CONSISTENT with what we should see on a flat earth model.
A SINGLE observation MIGHT appear consistent on first glance (“flat” horizon) but empiricism dictates that ALL of your observations or at least NEARLY ALL of your observations must be consistent with one another. The flat earth model doesn’t even come CLOSE to withstanding this kind of rigorous scrutiny.
Furthermore, that same body of observations is CONSISTENT with what we might perceive from within a globe model, but that isn't the point - we're not defending or supporting a positive claim for the globe earth, or attempting to "prove" anything about it, we're just making these three assumptions and challenging them for consistency.
So let's assume we're a flat earther and we believe the first assumption. When information is presented to us that contradicts the first assumption, that DOESN'T mean that we have to ALSO disregard the second and third assumption. We could instead say, "Well, that makes sense, but I still don't believe in a globe earth. The conspiracy is real, and that might not tell me that we live on a flat earth, but it does give me pause to SUSPECT the globe model."
What we end up with is something called argumentum ad ignorantiam - argument from ignorance.
Without relying on the conspiracy, all a flat earther can do when presented with the body of evidence that contradicts the first assumption is claim that we don't have enough information to PROVE a globe earth model and DISPROVE the conspiracy.
What you have to understand about this flawed reasoning is two things:
A ) Yes, the fact that most if not all of the circumstantial evidence is CONSISTENT with the globe earth model and INCONSISTENT with the flat earth model DOES lend empirical weight to the globe earth model, even if it doesn't "prove" it in the same way you might "prove" the existence of your own penis by looking down at it. To say that we don't have enough information to give us reasonable and accurate model for reality is to retreat into argumentum ad ignorantiam.
B ) You don't HAVE to disprove the conspiracy in oder to show WHY the circumstantial evidence is consistent with one model or the other. If the earth is flat, conspiracy or no, the fully scrutinized and compiled body of evidence will show that. It does not.
What you have to conclude from this if you're a flat earther is, "Well, even if I concede to the globe model, NASA/the government/science/Bill Nye the Science Guy are still lying about certain aspects of it, I just don’t know WHY."
This is again argumentum ad ignorantiam, as you are alleging that someone is lying without providing a logically consistent and reasonable motive to ascribe.
tl;dr...