Thats the thing, it isn't like they just take their combine performance, or just take the game tape... It is a collection of those things that they probably have a sliding scale for that allows them to rank the players...
Lets say you have two players at the same position who kick ass in game tape... One runs a 4.5/35 in vert and the other runs a 4.2/40 in vert... It makes the decision a little easier when you have to draft...
You can't coach athletics... Coaches like to see, which players are just natural and fluid in their movements, etc... The combine also shows how well players follow directions and how well they do under pressure. Mistakes that they make on tape can be coached... Combine shows things that game tape may not show.