With abombs for weapons systems design they needed people who just got huge kicks out of explosions (not kidding here). I guess it’s partially about personal internal motivations, and it might be more of a chance wether the thing you are intrinsically motivated to do falls under engineering or science (in both cases you get the feeling the greats did stuff they wanted to do regardless of the categorizations applied to their discipline - you get more capital affinity in engineering ofc).