Why is aircraft marshalling still necessary?
There are automated systems that provide the same function as a marshal. They are in place at some airports but not all since upgrades cost money and not all airports may see a use for it. 

At smaller fields marshals often serve multiple roles aside from just guiding aircraft they may also handle baggage, pump fuel, drive tugs and all the other various ramp jobs. In this case it may be more cost effective to keep a marshal around than instal a costly automated system that only replaces one of their jobs.