Wait they are that dumb.
If you have a small cheap telescope you can easily observe the phases of Venus. And planets aren't wandering stars.
In Flat Earth movement the main problem is denial.
Not only on this forum.
From time to time things here are pretty moderate.
Elsewhere could be even worse.
But high percentage of Flat Earthers still try to act (or at least look) civilised most of the time.
You can present them with the simplest and most obvious set of proof, they will deny it.
It is not question of facts and figures.
They stubbornly run away from the truth.
Their belief is a matter of their "strength in faith" and has nothing to do with general or even basic knowledge.
Math and other sciences are "devil's business".
If they can, they will twist facts, giving alternative "explanation" for all they could.
(Not to speak of total inconsistency of all those "explanations" when you try to incorporate them in one, single model.)
If they can't, they will sweep them under the carpet, and if they can't that either, they will call the whole world bunch of liars and conspirators.
Their problem is not in lack of understanding of physics, astronomy, navigation, geodesy, meteorology, telecommunications, Space exploration and other exact disciplines.
Their problem is in psychology, with roots in religion misinterpretations, and in work of mainly two charlatans: "Dr" Samuel Birley Rowbotham, and soon Wilbur Glenn Voliva.
(Voliva was FE tyrant in Zion, Illinois, and gained significant amount of money from it.)
Their tool is mostly cherry-picking, (failed) discrediting, and sometimes sparks of other types of attack.
Recent rebirth of Flat Earth movement was strongly supported by new wave of charlatans:
YouTubers that twist and juggle with "facts" (and add their own) to muddy the water and increase views and subscriptions for single main reason: money.
And with it probably for possibility to get "famous" in the minds of their naive victims.
Purpose of revealing facts here most likely would be not to let naive newbies, mainly kids, to fall for FE deception.
Unless they really want to on purpose.
There you can't help.