It's not though. There are countless examples of NFL QBs being successful initially because they are different types of players than the NFL is used to seeing. RG3 and Kaepernick are examples off the top of my head. NFL defenses did not know how to defend them. However; with film study, defensive coordinators caught on to the game and forced them to do things they were not capable of doing. In the case of RG3, who had a strong arm and was accurate for the most part, his biggest weapon was his legs. Once injuries hit, his perceived strengths outside of running weren't as strong because they were not complimented with his legs anymore.
Hell even Derek Anderson had pro bowl level of success early with the Browns because NFL defenses were not prepared for him. Once they got enough film and were able to game planned for him, the 2nd half of that season and the rest of his career was pretty bad.
Lamar Jackson can improve sure. However, defenses will game plan for him more effectively as well. That's why I have such low expectations from him.