From what I see, as a compulsive zillow-with-24hr-filter-scout, the market is a lot less liquid than one might believe. There are a lot of economists' $20 bills laying around. (Especially during COVID, but also generally). Even some $100 bills. And they almost always sit on the market for at least 12, if not 24 hours.

My explanatory theory is that people who are very interested in getting great deals on apartments are also the same kind of people who sign long-term leases.

IMO, the biggest barrier to arbitrage, that allows these good deals to float on the market for several days, is that most people are not interested in pouncing on an apartment that's come on the market <8 hours ago. Breaking your lease (the rental market equivalent of "selling", a requirement for "selling up") is just way too expensive.

Brokers (the people who are ostensibly paid to arbitrage) are also not super interested in getting their client the best arbitrage deal, because their commission will usually come out lower.

All this to say: next time you're moving, consider hounding zillow in the mornings and evenings every day, with a 24hr filter, in the 5-6 weeks prior to your move. Your patience will usually be rewarded.