I said something like “because canon almost always screws up canon het,” which I acknowledge is not a super helpful answer, but it was also a very small text box.
So just for fun I sat down and listed out ( all the ways canon screws up canon het. )
Which isn’t to say I never ship canon het. You can usually sell me on it in fanfic if you try, and I usually don’t actively anti-ship it or anything, I just don't get invested. And I haven’t been watching most of the recent shows that are supposed to be really good for canon het (like B99). It’s just that canon het is very rarely going to be the ship I get super invested in or go looking for. Often I read the fanfic until they become canon and then re-read the old fanfic from before they were canon.
But here’s some canon het I am 100% there for, just for fun:
( Not comprehensive! Just the first few I thought of! )