Here's why:
* Auxiliary verbs help the main verb to form tenses, moods, and voices. They work alongside the main verb to convey complete meaning.
* Main verbs are the action words in a sentence. They describe what the subject is doing.
Examples:
* I can swim. ("Can" is an auxiliary verb, "swim" is the main verb)
* She can't go. ("Can't" is an auxiliary verb, "go" is the main verb)
In short: "Can" helps to create a sentence structure, but it doesn't express the primary action or state.