By "cost of switching jobs" I mean that there's a negative cost in the long term to having a short job on your resume (especially if your previous job was also short), to not having an enthusiastic positive reference, to maybe getting a reputation as someone who doesn't want to stay at a job, etc. If I take a job paying, say, $100K, and someone is offering me $120K after six months, waiting until I've been at the job a year or two is technically costing me $10-30K over jumping right now but is probably worth more than that in the long term just to not have that six-month stint on my resume and to not burn bridges with my current employer.

After a couple of years it becomes more defensible to leave, so there are mechanisms for giving you significantly more money after several years (promotions, promotions to manager track, refresher grants, etc.) but fewer mechanisms for giving you more money in the first year or two, and basically none for within the first year, because employers know that even if your market value is theoretically higher, you can't actually take advantage of that yet.

I think that even in markets where job-hopping is high in general, any particular individual can still only job-hop so many times before it backfires on their ability to get a high salary.