1) newJob pays more than oldJob
2) oldJob is becoming unstable, toxic, and will likely be firing us all in 6 months, so better to leave early and start the clock over
In a world where we remain at oldJob til the end, we risk 6 months of that extra pay differential, plus we now have to job hunt while technically not being employed
Bringing this back to shaker8's point about financial risks, I think these kinds of job changes mostly point to systemic resiliency, not fragility.
But imagine if the US job market changed to this scenario: still the pay is high, still the expenses are low, still the jobs are available, but you have to change jobs every week. That would signal two things:
1) Employers are suddenly finding it advantageous to rid themselves of workers on short notice
2) Time and money are being wasted interviewing people and onboarding them rather than letting them work for sustained periods
I think such a system, while still "healthy" because workers are still employed and paid similarly, is "less healthy" than a system where the workers can expect the duration of their work at one employer to be longer than 1 year