Because that would wreck the game. Although the current luck based system should be altered to reward those who complete high level missions with the best gear.
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In one strike mission I had the highest amount of kills least amount of deaths and most precision kills I only found one decoherent engram my friends that were laying back and supporting both got legendary engrams as well as several blue drops so either increase the rate of drops or allow those that are lucky and get them all the time the option to sell them to the less fortunate so that others can keep up