I think it's because "shit" has acquired additional connotations, such as

1) An expression of dismay 2) Another word for "things"

"Poop" bypasses that and can therefore used to refer exclusively to feces in a nonmedical context.

… what do you call it in casual adult conversation?

shit, crap, faeces, roughly in that order of frequency, somewhat dependent on context.

It seems like a universal term in my experience.

In the US? I know many US people think their local culture is 'universal', and its corporations are trying to make it so, but have failed thus far. I've never heard a grown-up use 'poop' other than when talking to children. The UK equivalent of 'poop' would be 'poo', but even amongst the most curtain-twitchy suburbanites, I don't think its use between adults would be common.

Well it's the only word that is not a) "offensive" or crass (shit) or b) weirdly scientific (feces

I don't think this is a US thing really.

"Faeces" is very commonly used in medical conversations, which with an aging population in most of the developed world, represent a substantial proportion of human talk. "Crass" is a silly term to use for a fine aspect of our linguistic heritage (scatalogical words have a fascinating history) - and particularly daft if you're comparing it to the childish (and very American) 'poop' (which will no doubt itself eventually be subject to the euphemism treadmill and be thought of as 'crass' by the next wave of neo-Victorians). 'Shit' is by far the most commonly used word for this topic amongst grown up speakers of English that I have known.

Well sounds like you aren't hanging out with the classiest crowd then

Well if your definition of 'classy' excludes from worthwhile companionship thousands of people of the 4 nations I've lived in, classes ranging from street kids to multimillionaires, co-workers in shops and factories and corporate offices, ages from the primary school kids I've taught to the 90 year olds I've been friends with, then you're working from a sadly narrow social palette.

Perhaps your only experience of life is middle-class US sub/exburbia, but, truly, that group's local mores do not represent the universal cultural apex you think it does.

She's American. I was asking if this is an oddity of US culture. To my (non-US) ears it just sounds childish, like saying 'yummie' or calling your parents 'mummy/daddy'.

You would hate my country's language then. Diminutives are used in everyday conversation by adults, for no reason other than it sounds more fun.

What language? Sounding fun and inappropriately childish aren't necessarily the same thing - though language pragmatics are vastly complex and are easier to embody than really explain. Anyway I'd probably be more curious than judgemental about a language I'm not a native speaker of.

English is my own language, which is a different matter. The locution sounds inappropriately childish to me, so I was wondering if this was just the norm in the US (which often is oddly prim in some ways, wildly licentious in others, compared to some other English-speaking places).