The reason is that node.js uses an event loop for processing, therefore if all you're doing is initiating a single node.js process without anything else, it'll not be multi-threaded and thus not use more than one CPU (core).
However, you can use the node.js cluster API to fork the node process so you can take advantage of multiple CPUs (cores): https://nodejs.org/docs/latest/api/cluster.html. If you write your code that approach, then having additional compute units will assist you.
There is one caveat, in which EC2 compute units are detailed per instance. For some instances, you can get more "compute units" per virtual core. So if you choose an instance that has two compute units per virtual core versus one that has one per core, you will be able to execute node on a CPU that has more compute units. However, it's like after 2 compute units the computing power is split per core which suggests you will not get any benefit from the multiple cores.