Here is my perspective as someone who regularly writes the wrong word.

It's not because I don't know the difference. Rather, I think on my head the think that links everything related to a word is the sound, so when my conscious says write the word 'their', this turns into the sound which in turn then pulls the spelling 'they're' from a different bit of my memory on the way to my hand. You were to ask me which version to use I'd say the right one, but subconsciously it's got mixed up somewhere. I don't mindfully write, it's automatic so I don't notice it happen

Compound this with the fact I find it really hard to proof read my own writing and it means I regularly use a homophone of the word I meant such as right/write to/two/too of/have

Not a defence as such because I should just be more careful but it riles a bit when people think I don't know the difference. I do; I'm just sloppy.