Also, each play is rated by several different scouts, not just by one person. It's a consensus opinion of trained, unbiased (as unbiased as a human can be) scouts.
Don't the eye test people believe in scouts? It is scouts who are supplying the data, often times former players or coaches.
Don't disregard the scouts' opinions just because their opinions are then put into analytic form.
Besides that, there is much more that goes into it besides opinions.