Do you not see the irony of forcing yourself on other people, despite their wishes, and justifying this by saying that they're too self-involved?
You are sitting so close to someone that parts of your bodies probably touch, you smell them, you feel them, you hear them. The one doing the forcing with all that is the evil aircraft company, and though it's customary to regard such forced close encounters as "non-spaces" by pretending that no, you're not crammed in with a stranger for hours and hours, the reality is that you are.
The question is how you react to that, and offering to acknowledge the presence of the other and to find out their wishes regarding the flight is the common sense thing to do. Like pinging a server, if you will. If you don't ask, you won't find out.
Well, if there are non-verbal hints (looking away etc), by all means, stay quiet. However, you probably clearly notice that a protocol which forbids offering to start a conversation would result in countless acquaintances and friends never meeting, even if both may have preferred conversation.
In the end, even to an introvert, simply stating "Oh hello, I'm so and so, unfortunately I have a lot on my mind, I'm sure you understand" isn't outside the bounds of the reasonable. Do you disagree?
FWIW, I am an introvert, and I agree with you. I have no desire to start conversations with strangers on the plane, but I understand that extroverts do. I refuse them politely along the lines that you suggest here, and nobody has ever thought me rude because of it. (Or if they did, they were polite enough not to say so.)
r/Fitness does a weekly "Moronic Monday", a judgment-free thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. I thought this seemed like a useful thing to have here - after all, the concepts discussed on LessWrong are probably at least a little harder to grasp than those of weightlifting. Plus, I have a few stupid questions of my own, so it doesn't seem unreasonable that other people might as well.