Here's why:
* Gerunds are verb forms ending in "-ing" that act as nouns. They represent the action itself.
* Nouns are subjects when they perform the action of a verb. However, gerunds are already the action, so they don't need a separate subject to do the acting.
Example:
* "Running is good exercise." Here, "running" is the gerund, acting as the subject of the sentence. It is the action itself, not a person or thing performing the action.
Instead of subjects, gerunds can be:
* Objects of verbs: "I enjoy swimming."
* Objects of prepositions: "I am good at cooking."
* Subjects of verbals: "Singing in the shower is fun."
If you're looking for an action performed by a subject, you need to look at the verb in the sentence, not the gerund.