What doest it mean: "computer is an intelligence amplifier"?
Let's first clarify the scenario. What you're basically asking is why it is more likely to randomly write a computer program that outputs the works of Shakespeare than writing the works of Shakespeare directly.

The simple answer to this is that there are many ways to write a program that outputs a fixed string, but only one way to directly write this string. This may sound a bit strange, because there is clearly a mapping from strings that describe a computer program that outputs a string to another string (that is, the string outputted by the described program), and we could interpret a large amount of strings as descriptions of programs that outputs something. So why are the sizes different?

The reason is that these mappings are not bijective: the map from programs to strings outputted is clearly surjective, as we can output any string, but also clearly not injective, as there are multiple programs that output the same string. So, the set of programs must be larger than the set of strings outputted by them. That means if we sample over the set of programs, we need to sample over a much smaller set to get strings of a certain size, than if we were to sample over the strings directly.