Due to how the written word "you're" sounds a lot like "your" uttered in sentences. Chances are they don’t really think about it, or rather. Chances are that when they are writing, they are constantly thinking about what to write; while continuously writing. Meaning, that some things get out of context, so when your brain tries to process the word "you're", and you aren’t utterly focused on the context, it might confuse the brain enough to accept and write "your" instead, which is if spoken, just as good. So, your "inner" voice reading your thoughts is happy, and so are you, as you're none the wiser
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Truth be told, it's overall an utter bastardisation, which could have been avoided by forcefully preventing contractions. But yea, informal use of the language creates a certain type of acceptance towards ignorance.