In a sentence like "La tarte que j'ai faite", why does the past participle match the gender of the preceding subject noun?

In French sentences, when the past participle comes after the subject noun, it always matches the gender and the number of the subject noun.

For example, in "Les rendez-vous qu'on a oubliés", the past participle "oublié" takes an "s" because "les rendez-vous" is in plural, and the past participle needs to match it. Similarly, in the sentences "Les photos que nous avouns prises" and "Les filles que vous avez rencontrées", both the feminine gender and the plural number need to appear at the end of the past participles because of the preceding subject nouns.