Why don’t we just use radio to communicate with the ramp marshall instead of all the hand signals? They could just say move a little left or right, stop etc instead of learning all the hand signals.
There are several good reasons:
Hand signals are actually very easy to learn.
Reacting to a visual signal is instinctive and doesn't require knowledge of any spoken language.
Batteries don't run out on hand signals. (although at night a lit wand could die...)
You will never run out of radio frequencies with visual signals.
You will never have radio interference from somebody else transmitting on the same frequency.
You will never get stepped on by another radio or intercom transmission.
In short, there's no compelling reason to fix a very reliable system that isn't broken.
With hand signals, the pilot and marshal know exactly who they are communicating with. With radio, you'll need extra protocol so that they know who they are talking to. On a shared radio channel, that means callsigns with every transmission. With individual (non-shared) channels, they still have to figure out which channel to use and exchange callsigns at least once.
Skip these steps, or have them go wrong, and eventually two planes are going to be moving at the same time, each being monitored by the opposite plane's marshaller. When the marshal makes an urgent call to stop and the wrong plane stops, it'll be a bad day for everyone.