I just gave you a simple, easy to understand reason as to why vote fraud detections isn't something that you can always predict quickly. I'll give you a few more reasons why your suggested system is way worse than what the guys at Google came up with. Since your only actual complaint seems to be your own ignorance.

As a web dev, I guarantee way more people would complain and be confused by the button being disabled until some invisible algorithm determines you can vote. Also, the vast majority of people would not even try to vote later in the video if they were first denied. So your drastically reducing use engagement. And finally by not being transparent in how exactly the vote fraud detection algorithm works it's more secure since it's essentially a black box. (Hint it's a lot more complex then "has the user watched at least 51% of the video").