I hear you, man. I have a couple theories on why.
1) Sometimes people approach debates just to validate their own opinions, instead of actively seeking what is true. This is not the same as proposing a theory, but they want their opinion on that theory: "this theory is true" "this theory is false" validated by other people. This is because, deep down, they are uncertain about whether their opinion is true, but they want it to be true so much that they will ignore everything to the contrary.
Truth is dang painful, especially when you have an emotional stake in it. But no amount of my (or yours, or anyone's) emotions will change the truth. Period.
2) Having uncertainty about the major truths of life can cause people to have emotional stakes in trivial items. This is because, when you're uncertain about anything, the amount of confidence in your thinking about the uncertainty (which is huge, major, not going away) depends on the validation of the accuracy of your thinking at any given moment. This means that being uncertain - not being sure - of the major truths of life - causes you to seek validation that you are still thinking correctly, because whether or not that big thing you want to be true is true depends on your thinking about it, because only your thinking and reasoning about it makes it true! And if you're not thinking correctly about this tiny little minor thing, could you be wrong about this huge major thing too? Ooo...scary. Terrors! Quick...I'm not wrong about this tiny little thing because...
On the other hand, if the opposite is true, that you are absolutely certain of the answers to life's major questions (Why do I exist? Where do I come from? etc), and that certainty comes from something greater than yourself (logic, perhaps), then you will let the little things that you are wrong about pass. They are learning expieriances - the little debates don't change the big things because you know what they are, and being wrong about this little thing isn't going to change who you are, or how right you are about the big things, because you know you are right about that. Period.
I could go on and on about that, but I think I've either accomplished my goal or confused the living daylights out of you all by now.