...conspiracy theory commencing.
How do you know that aircraft follow a curved path? Because they tell you so?
Well, um...... since my father's an airline pilot and I've seen the maps he studies, yeah.
Besides that, it's just simple logic. Why else, other than to save money on jet fuel, would airlines pass through certain layover stops that are not directly on a latitudinal line? Do they do this for fun? Have you ever flown across seas before? Because I have and I can say with absolute certainty that I've passed through countries that are not directly east or west, but rather slightly north or south.
So let me answer your conspiracy theory with another question;
How do you 'know' that your skull has a brain in it? Because people just tell you so? For all you know, there's just a giant gray piece of play-doh up there. I mean, you've never cut open yours or anyone else's head and seen first-hand what a brain looks like, but you don't go around touting that it's incorrect simply because you have never had subjective experience of seeing your own brain. You kinda trust what other's say in certain circumstances based on a variety of information about your belief systems.
You can have conspiracy theories about anything in the entire world because there's no way that you could possible experience first-hand everything that you are told about. Sometimes, it simply makes sense.... and unless you believe that everyone else around you is a programmed robot, trusting what others say can be beneficial.
This is ridiculously elementary.