Because a child that went off the derech and left behind frum parents and siblings, frum friends and went off the derech not because he “went to” a better life but because he or she was running away from something hurtful and painful will not argue with you that the life they are now leading is “better”. They are not going to argue with you at all.
Mr. aries, OTD’s argue ALL the time. Please, while there are those that do want to come back they who won’t argue. those who don’t want to return to Yiddishkeit will repeatedly say how happy they are with their current lifestyle, that’s it’s much better and they’ll continuously argue about Yiddishkeit even if their arguments make no sense.
They might not argue with you because you respect them and accept them the way they are, with their current actions so why would they even argue with you?