Stories about childhood friends who grew up together just come off to me like the author wants to write incest but is afraid of the stigma. This may just be me but I don't think I can be attracted to someone I've known since childhood
That's a stretch. Especially when there are quite a few incest stories out there and also the ability to do step siblings if they really want to sort of do incest without as much of a stigma.
I do agree that incest and childhood friend stories have similar tropes. One of the problems that generally pops up in childhood friend stories to create drama is one or both parties only ever viewing each other as basically brother or sister. But the difference in these stories are that incest has to stay secret while a relationship with a childhood friend can be public. Childhood friend stories often have other people wondering why the two are not dating already. That doesn't pop up in incest because the society would find it wrong. If someone does comment on two siblings being too close it is done in a way to point out it is weird. In this way there are different scenarios that can be played out through incest or childhood friend stories.
Basically, if someone wants to write incest, they'll just write incest. Pseudo or real. Childhood friend stories have enough difference that they are written for the childhood friend purpose.