Why isn’t there a non religious equivalent of agape love?

The noun agapē first arises in Koine. (In fact, the first attestations, other than as a proper name, are in the Septuagint.) But the related verb agapaō was used for 800 years before Christ, both agapē and agapaō have been used for 2000 years since Christ, and there’s nothing intrinsically Christian about agapē.

In fact, the Church definition of agapē as “self-denying, divine love” is, well, it’s an eisegesis. In reality, agapē is just unmarked love; and philia was likely the more unmarked term in Classical times. Look at ἀγάπη, DGE Diccionario Griego-Español, the latest big Greek dictionary:

1. Sexual love, with a link to Song of Songs thank you very much. (Inb4 “no, no, there’s no sex in the Song of Songs, it’s all metaphor.)
2. General sense: love, affection
3. Religious sense: love between god and humanity; Christian love, charity
4. a favour; alms
5. agape, communal feast; funeral feast; church, community