English needed a name for the hardest thinking there is, and it picked this job. So the phrase points two ways at once. One way it points at real people, in real buildings, working out how a rocket flies. The other way it points at nobody in particular: a standard of cleverness held up in the air so that the speaker can say you do not need it. That second direction is where you will meet the words most of the time, and the joke only works because the first direction is real.
From the word’s history
Wiktionary records the phrase simply as rocket + scientist. The compound is literal, and the joking use grows out of what the job stands for rather than out of any separate root.
Wiktionary 把這個詞記成 rocket + scientist，就是兩個字直接相加，字面上沒有第二條來源。至於那個開玩笑的用法，可以想成是從「這份工作代表什麼」長出來的，而不是另外一條字源。
Wiktionary, "rocket scientist"
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