Because the next highlight tape that is less than flattering will be the first one I see.
I watched a ‘highlight’ tape of a kid who was maybe 3rd string, by the time he got in the game it was out of hand and all deep subs were in, guy looked like a monster. Funny stuff.
They are, for the most part, without the necessary context to make an assessment. Don't have enough information about who they're playing against, what's the caliber of the skill players they are playing with, etc.
Fun to watch a guy's best throws, but really not worth much in evaluating what they are going to be able to bring against B1G competition.