stick a tree up
For some people it's not that easy. They live in really small,shitty,damp ,mouldy houses/flats and the whole stick a tree up involves a lot of work. Moving furniture around, clearing up the mould (they do anyways but now there's the extra special Xmas one, so cheerful) ,trying to find space for everything etc. That's without the fun of opening the bag and finding the xmas tree ruined , or the lights or the wrapping paper or sometimes all of it because of the aforementioned damp and mould. It's depressing and fucking exhausting.
Just stick a tree up indeed.
attend a Christmas concert if there is one
Some schools have a nativity, Christmas concert, Christmas fair,secret santa, Christmas jumper and so on. Times that by 2 or 3 kids. There's extra shit to buy or make and to remember.
Just a roast
Sure if you have a small family and never host or you only do so for a couple of extra people. Not if you're actually feeding 10 people, even in terms of strictly food just a roast is not enough, and let's face it there are expectations in general, especially so at xmas. Let's not forget all the MIL fed me two roast potatoes and a thin slice of chicken threads during the years.
Then you can have all kinds of combinations and extra stressful combinations, arsehole families (but they're too entrenched in FOG ), kids or parents with SEN and or disabilities, sudden illnesses, jobs that are 100% busier and more stressful around that time of year, separated parents with acrimonious relationships and a billion other things.
My Christmas is mostly like yours so while not pressured, I do get a bit stressy and busy but not enough to really affect me. However, I'm not that blinkered or sheltered to just assume everyone's is the same and any stress is simply of their own making.