At the end of the day, it's really pretty simple. As a prospect, he's not elite. He doesn't have many if any elite traits. Then you factor in that to many people, he comes across as cocky/arrogant/etc., it becomes an issue for teams. You have a QB such as Caleb Williams that was "cocky" or whatever term someone would want to use. However, he had ELITE traits. When you don't have elite traits, you sure as hell better come across to teams as having a great attitude and minimal baggage, and Sanders does not check those boxes.
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