The reason is that the Striker subclass is built for Close Quarters.
Their super has extremely close range compared to the Nova Bomb and Golden Gun. A little bit of toughness (they aren't unkillable mid-super, just a lot HARDER to kill) might just be needed to keep the super from being useless.
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Well I mean if you see a warlock winding up its super you can just shoot them down or even if you avoid the blast you can I think what I experienced was just a glitch because i know it happened as I recorded it on xbox one but others have tried and succeeded at killing them mid super so maybe a random occurrence but what you say does make a lot of sense.