If you've ever reffed a game (which I guess you haven't from your post), things that look really obvious from one angle, don't look clear from another one.
Changing a position by even just one yard gives you a completely different perspective on things and like players, referees make mistakes. Usually though, the players don't get abuse for the mistakes they make.
I said penalty as soon as it happened but just because I saw something from the TV angle, doesn't mean the referee had a clear view of it from his. VAR has been brought in to help eradicate these human errors and whilst it has its problems, it's getting the right result the majority of the time