We always have to be careful when we think we know something. Many accomplished professionals or master tradesmen will tell you the more they know about their craft, the more they recognize that they know very little in relation to what there is to know. If humility is best learned by those who know the most, then we would do well to admit that there is much for us to learn about living the Christian life. That's why there is only one place to turn for advice, the Bible. What does God say?
Why did God say that if He thought it was OK to think about, watch, be entertained by and even be immersed in violence, greed and lust for hours at a time?
Traps are usually hidden. Things that look safe can instantly snare us. We have an enemy and he is a master trapper and deceiver, he's good at hiding his traps. There are so many things in movies that CAN influence you towards a life of violence and debauchery at some level. Whether instantly or gradually, how do you know you will not be swept up into it?
There are bad winds blowing around out there trying to deceive and destroy us Eph 4:14. If you put your sail up to catch them, you're going to be tossed around!
Usually, we mean for it to be "just entertainment", just good clean fun. But movies are designed to sweep people away, to transport you into the story, sometimes for the entertainment value and sometimes to sell products, but each movie, TV show or commercial usually includes a "worldview" that is not Christian and often not even moral.
You may think you can handle it, but we have to admit that just as in every area of life there are many things about which we know very little, we could be wrong about this. Just because you can recognize the overt acts of sin in a movie doesn't mean you are safe from being influenced to commit them. Seeing it all the time "de-sensitizes" you to the sin, hardens your heart a little at at a time, makes you "get used to it". The devil is a slick con man, and he wants to subvert the truth in your heart, you may see the obvious problems in movies, but that doesn't mean you know anything about the traps and snares that are hidden it. God advised against watching or even thinking about things that are not lovely good and pure.
Life is too short to take the chance, compared to eternity our life is a" vapor". We don't have the time to gamble with trivial pursuits that could steal our lives. If we choose to go against God's advice, we might survive spiritually, but there are no guarantees.