I find it weird that people in Isekai can be so detached from religion or the absence of it and not break down after realizing that they have been reincarnated or be traveled to a completely different plane of existence.
Has anyone else wondered why nobody has really used that as a plot point?
Because it's messy.
It's the same reason authors gloss over or entirely skip "civilized" men and women having to kill another person, let alone a monster (see
Grimgar of Fantasy and Ash for a series that does explore this angle to heartbreaking effect), it requires a certain level of emotional awareness to pull off well and some authors may think it stands in the way of the fun portal fantasy romp they want to tell, which admittedly, it might.
A possible solution would have the characters either embrace a religion they can see after a paladin invokes the name of a goddess, or after said goddess embraces the character, but that would only work cleanly for people who are already not religious, at least not seriously religious. Another solution lies in a character clinging to their religion as a safety blanket, something to comfort them in the face of the trials to come, perhaps going on to proselytize their religion and in some narratives,
constructing their god in their new world through the force of their faith alone
40K style.