I agree with the rest of your comment, but in my experience landlords in hot markets can be pretty capricious. If they know they'll have a steady stream of applicants, many will definitely reject qualified tenants based on personal whims or to hold out for someone they see as "more" qualified or more likely to stay long term. That said, as long as the apartment still gets turned around quickly then it's not really a threat to this business model.

In NYC, it's usually required for a subletter to take over the remainder of a lease and then have the option of signing a new lease. The option to simply immediately break the old lease and start a new one usually doesn't exist. (Or if it does, it's only with a penalty for the lease-breaker.)

I don't know why this is, however -- if it's for administrative convenience, legal reasons, or financial reasons. But your idea that "the landlord can agree to the transfer of a lease", well it's true they can, but as a general rule they won't.