Probably because there is no requirement. I have written specifications for several hundred elevators over my career and never once was I asked or required to put steel mesh between hoistways.
The only reason I can think of such a requirement is for security reasons. Some elevators within a single hoistway may serve floors that have restricted access. To prevent someone from accessing a restricted floor from a non-restrictive floor without adding the cost of a full wall, some may use metal mesh. Think of an elevator used in courthouses where one elevator is for the public, one for the judges, and one for prisoner movement. They may be within the same hoistway for cost reasons, but you do not want prisoners in one elevator car to be able to access another car within the same hoistway and escape.
