Landlords can ask for whatever they want within the law, and people can either accept it or go elsewhere. New York, apparently, has no law limiting security deposits. [0]
> Who the hell has 10k lying around just to rent a place?
People (or, perhaps, e.g., their parents) renting in (at least certain parts of) New York City, apparently.
[0] Several other jurisdictions do; California, for instance, limits them two months rent for unfurnished units, three months for furnished units, and increases the limit by half of a months rent for if the tenant brings in a waterbed -- and note that "last months rent" is treated as part of the security deposit limit, so if you break it into "first, last, plus deposit" the deposit part is limited to one months rent for an unfurnished unit, etc.