Because the people who use the phrase life admin are using it as a catch-all term to mean everything that isn't paid work or social stuff.
EVERYTHING.
I don't actually use that phrase - I just call it tasks or chores (to myself) or stuff if I'm talking about it to someone else. As in:
"Wordler, want to come out for a drink after work tomorrow"
"Thanks, can't this week I have too much stuff to get done at home, how about next week"
That week's stuff could be getting one of the kid's passports which would mean - getting new pictures,filling out the forms, getting to the post office to have them signed in front of the officials there etc.
Or it might be I have to host a dinner party for my husband's work colleagues and I'm menu planning and doing an extra deep clean on the house (yes, he's helping too)
The next time my 'life admin' gets in the way of something on my 'social' list it might be an elderly relative needs taking to hospital and needs 2-3 days of visiting and hand-holding.
I'm calling it stuff if I need to explain why I'm busy - someone else is calling it 'life admin'
Fair enough - call them out for a wanky phrase but you can't claim that everyone's chore list is the same. And that every task presents and is completed the same way or as easily by different people.