1. They tend to work double jobs at frankly lower tier companies with lower standards and significantly lower pay. Because the companies are lower tier, they stagnate in their career growth because the company is not teaching them good skills. And you cannot put both jobs in your resume, only one on top of that, and this will show up in future background checks.
2. Because of double working, they are pretty much guaranteed to not get promoted beyond the standard terminal level.
3. They think that promos (like this article), are at most only %10, while promos are more like a x1.5 to x2 of your income.
4. You cannot work at proper startups and learn a ton, because the workload would be way too high for this strategy.
Real example: One person I know has 2 $150k jobs for a total of $300k. If they joined as a jr engineer at a FANGMULA or equivalent, they would be making $300k, not be fucking stressed about the duplicity that is working 2 jobs, learn more because it's a better company and get promoed to sr engineer within a year or two and make $400-500k instead. If they have ambition, they can cross the leadership rubicon (either through becoming a staff engineer or manager) and make even more, reaching up to $700k-$1M eventually.
Maybe if your having a hard time breaking into startups or FANGMULA and your just starting out, this might be an ok strategy, but beyond that, it's not a good idea.