Very good point. I had never done mistakes like these before moving to the US, and at the time English was not my primary language. A few years later, I make these mistakes more and more often.
I tend to blame this on the fact that, at the beginning, speech was less natural than writing (I was writing scientific articles before moving to the US) and I made several non-native-speaker mistakes (rape/grate and he/she) in writing. Now I am making fewer of these but increasingly more of the "it's/its" type, possibly due to speech becoming more of a natural act and therefore driving my writing. Because "it's" and "its" sound the same, I might overlook it in writing too. "Should of" and "should have" seem to fit in this case as well.
I tend to blame this on the fact that, at the beginning, speech was less natural than writing (I was writing scientific articles before moving to the US) and I made several non-native-speaker mistakes (rape/grate and he/she) in writing. Now I am making fewer of these but increasingly more of the "it's/its" type, possibly due to speech becoming more of a natural act and therefore driving my writing. Because "it's" and "its" sound the same, I might overlook it in writing too. "Should of" and "should have" seem to fit in this case as well.
[Edit: last sentence]