Here's why:
* "Am, are, is" are the present tense forms of the verb "to be".
* "Been" is the past participle of "to be", used in perfect tenses (present perfect, past perfect, and future perfect).
Examples:
* I have been to the store. (Present perfect)
* He had been waiting for hours. (Past perfect)
* She will have been working all day. (Future perfect)