My gut feeling is this doesn't entirely explain it, but any tenant is also more work than no tenant, so they may not want to do the work of signing a new lease and dealing with the work it will trigger for less than you're making per-head on your current tenants.
There's probably also a fear that if you let a new tenant in for lower rent, this might lower the rent other people expect and the problem starts snowballing.
Surely this works in a short timeframe where a landlord can make a tactical decision to let the property sit empty rather than rent it out for less than a threshold. Landlords can even collude on a minimal rent below which they will not rent. (Basically "hold the line"). But how is this sustainable in the long run? The money for the original mortgage has to come from some place. Not all landlords have an infinite supply of money to keep this practice going on for a few years. (There is no VC funding available for them).
With commercial units (eg - downtown San Francisco) it is even harder because with remote work, the jobs are never coming back to the city.
There's probably also a fear that if you let a new tenant in for lower rent, this might lower the rent other people expect and the problem starts snowballing.