My personal theory is that native speakers are so used to those words being similar that they don't care about the difference so much. For people who learned English later on it's/its are two completely different words. At least that's how I see it. I see native speakers confusing them much more often than foreigners I know. There's a similar issue with even more crazy mistakes... I've never seen a foreigner write "should of" ('ve) - but many people in Bristol do that all the time.