Single people only have to manage for themselves. Now imagine the difference in workload if you have to do all you have to do as a single person for multiple people.
For some things it doesn't make a difference. But, let's take dental appointments, for example. I only have to keep track of my own. I know if I've got toothache. If I forget or don't bother with a check up, it's my own teeth.
If I had, let's say, two kids, I have to remember when each of them is due a check up. If they miss it, it's my problem. If their teeth get in a state, they can't fix it, I have to do it. If they aren't forthcoming about being in pain (some kids aren't), I have to make sure I spot the signs. If they aren't good brushers, I've got to try to do something about that, because they are kids and I am responsible for them.
Yes, it's still 'just life' but it's 'just' 3 people's lives, 2 of which are much less capable than I am of making decisions to lighten that load.
If you have them got another functioning adult in the house who only looks after themselves, or, even worse, doesn't even do that, can't you see how different that is to being a single adult?
I'm not single, but I don't have kids and I can certainly see the difference just by not having kids in terms of my mental load.