Why there is eaten instead of ate in past participle of this word?

Answer 1

To eat is an irregular verb in the past participle which when combined with a helping verb (has, have, or had) completely departs from the normal conjugation and does something weird. Many other languages have these kinds of words as well.

If the past tense of the verb is not simply the addition of -ed the verb is going to be irregular. (For example "eated" would not be the correct past tense so the past participle is probably going to be something weird, too).

Answer 2

The past participle "eaten" is used as it follows the past perfect tense of "have eaten." This construction requires the past participle form "eaten" to indicate a completed action in the past. "Ate" is the simple past tense form used to describe a specific past action without relation to another event.