1.  It's rather difficult to do the common "Two Call System" when the guy has no cell phone to call from the parking lot or street of the providers incall address. Pay phones are getting more and more scarce and the provider also doesn't want to have to answer every "strange" call coming in during her wait for your arrival.

2. If either party is running behind schedule the ability to make a courtesy call to inform the other is moot.

3. Some apartment buildings have security cameras by the common/main entrance that documents every caller. They conceivably could be recording AND documenting each push of each suites buzzer while imaging the caller.. By having the client call on his phone while approaching the door the provider can allow his access in a more clandestine manner. Also; anyone who lives in an apartment with such a security system knows that those buzzers make the rudest most nerve-wracking sound known to man.

4. With all the hobbyists there are it is not worth the providers time to have to worry about changing their own protocols and security tactics to accommodate for a precious few Luddites to modern communications technologies.