Have you actually asked them why they do it OP?
Not in too blunt a manner, but you'd think that if they were going to be forthcoming with the reason they'd have supplied it when we asked if they had any dietary requirements, sent the list of what we'd ordered, and offered to add to it.
I mean, I always do this but I’m coeliac. Are you sure there’s no medical reason?
I'm aware of their other medical issues (and they have a broad overview of mine), so I'd be surprised if they had shared that medical information but not an allergy?
And, as I say, it's four adults, so all having the same secret medical issue seems less likely.
You mention chihuahuas on the floor not the worktop. It’s still animals in the kitchen. Things like fur potentially in the meal is off putting.
From chihuahuas that have been dead for years? They'd have to be haunting the place... the hosting household does sometimes have visiting dogs, but not full time resident ones.
Much of my branch of the family considers disliking dogs to be a major character flaw, enough to make you consider the person as a whole entirely suspect, so they did make themselves stick out like sore thumbs with those comments (the older members of that branch of the family grew up with dogs ffs!)
Are you ever invited to eat at their house and, if so, what happens? It does sound a rather odd scenario in that you admit you don't see them that frequently but can't not invite them to join you for whatever this 'occasion' is?
We never go to them because they live a long way away, and we only ever see them in the context of shared relatives. No one else in the family lives in their area, so they always come to us.
So, I've no idea what it's like at their house (though I can imagine that dust wouldn't dare to enter), and no idea if they do the same to other people, because I don't think I've ever been to a third party's house with them.
It sounds like you're upset because you're trying to justify that everything is more than adequate in terms of provisions, hygiene etc. I think it's really nice of you to show them the list of food being bought and offer to add items in but if they've said no and you like their company well enough that's kind of that. Unless they're actually sat there actively insulting you...
It's partly that - but also one of the ways we show we care about people is by sharing food - which is a really common theme across pretty much every culture worldwide, so I suspect it's part of what it is to be human.
Eating together, sharing meals, and (metaphorically) breaking bread is important to almost everyone at some level - so having that way of caring removed, when you've gone out of your way to accommodate, with no real idea why, feels very jarring.
As I think I said upthread, it wouldn't feel so weird if they brought some food they were happy to eat to share with everyone - but they seem very keen on gastronomic apartheid. They never, ever offer to share anything they've brought.
I have a relative who does this kind of thing. Yes, it could be because they don't perceive the hygiene around the food to be good enough for them but if it's coming from a place of phobia and control then it never will be unless they have the control. It's not ideal but there is a difference between turning your nose up at someone's hospitality and not being able to cope with something. My relative thinks even if someone doesn't have a dog/cat anymore then the fact they used to means they don't understand hygiene and even if they did they'd allow people into the kitchen who did and they would "transfer".
You're quite probably right that they hold such phobic / controlling beliefs - though as I hold qualifications in food hygiene and somehow manage not to poison the general public when I'm at work, you'd think it might engender a little trust!