Why I think of them is je pense à eux ?

Question

Why don't we use indirect object pronouns like je leur pense ?

Accepted Answer

Bonjour Winston,
As Maarten said, ‘penser à’ is part of a list of verbs that only accept stress pronouns to replace their indirect complement.
Here are more examples : avoir affaire à, être à (dans le sens d’appartenir), faire appel à, faire référence à, faire attention à, penser à, prendre garde à, recourir à, renoncer à, rêver à, songer à, tenir à.
Je pense à mes enfants → I think about my children
Je pense à eux → I think about them.
 
I hope this is helpful.
Bonne journée!

Answer

Penser is a verb which can take a COD as well as a COI:

Je te pense méchant! — I think you’re mean! (Te is the COD)

Je pense à toi. — I'm thinking of you. (à toi is the COI)

To keep the two apart, you can’t say je te pense and have te be the indirect object pronoun. Note, however, that for objects you can say j’y pense. But in this case, there is no danger of confusion because the pronoun y can never be a stand-in for a COD.

Answer

More information here - «penser à quelqu'un» is a 'special case' (exception) where an indirect object pronoun cannot replace «quelqu'un», and a stress pronoun is required.  https://www.cliffsnotes.com/study-guides/french/french-i/french-i-pronouns/using-object-pronouns#:~:text=The%20expression%20penser%20%C3%A0%20(to,they%20are%20followed%20by%20%C3%A0.
https://www.lawlessfrench.com/grammar/indirect-objects-2/ this also has explanation