Here's why:
* "There" is an expletive: It's a word that fills a grammatical slot but doesn't have any real meaning. It's used to introduce a sentence, often to delay the subject until the end for emphasis.
* True subject follows: The true subject of the sentence always follows the expletive "there."
Examples:
* There is a cat on the porch. ("Cat" is the subject)
* There are many stars in the sky. ("Stars" is the subject)
* There seems to be a problem. ("Problem" is the subject)
Remember: Even though "there" appears at the beginning of the sentence, it's not the doer of the action and therefore not the subject.