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<p>Probably because CS was a newer field then, so there were fewer people with bachelor’s degrees in CS, so there were more opportunities for career-changers (with no degree, or degree in an unrelated major) to move into computer software.</p>
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<p>Remember that the no-CS-degree people working in computer software have presumably self-educated the needed CS topics. Self-education typically requires a high level of interest, ability, and motivation, compared to learning in a more structured classroom and lab environment in college. People with that high level of interest, ability, and motivation to self-educate the base level of CS topics are much less common than those who can be educated in CS at college.</p>
<p>Of course, having a long career does mean continuing self-education.</p>