[AngelicaDeAlquezar]

Because "haber + past participle" is a conjugation by itself, so past participle remains unchanged.

Nosotros hemos nacido - we've been born
Yo he cambiado - I have changed
Ellas han venido - they have come
María ha cantado - María has sung
Vosotros habéis entrado - you have come in


Not the same case when past participle works as an adjective.

Las sillas pintadas - the painted chairs
Los árboles cortados - the cut down trees
La niña educada - the polite girl
El auto vendido - the sold car


When the past participle is a part of passive voice, it must also agree with gender and number:

La casa ha sido pintada. - The house has been painted.
Los perros han sido encerrados. - The dogs have been locked up.
Las canciones han sido cantadas. - The songs have been sung.
El cuarto ha sido decorado. - The room has been decorated.