Here's why:
* Regular verbs in English form their past participle by adding "-ed" to the base form. "Check" is a regular verb.
* Irregular verbs have different past participle forms. "Check" is not an irregular verb.
Therefore, the correct past participle is checked.
Here are some examples:
* I have checked my email.
* The package has been checked for damage.
* The teacher checked the students' work.