I've noticed this more with SAHM. Obviously I'm not talking about this from a general population level, but my friends, close friends who choose the SAHM path - there is a light that has gone. I think it's because there's just not respite from it and perhaps it's not just the kids and being a parent - it's the way some are treated differently by husbands, extended family and society. It's very easy to take someone for granted when they're 'always there at home' and you feel that.
The Mum's I know who are still working 3 or more days and 2 or more days outside of the home - they seem largely the same people.
On the one hand, where I used to love working - I've definitely developed a 'could care less' attitude. But being in the office? It's the one place where I'm still me. I'm not Jane's Mum. I'm the person I was before kids and I'm treated like that. My job is the only place where the pre-kid version of me still exists. I'm respected, I'm dealing with non-child related things, I'm having 'office banter', I can do what I want with my lunch hour without having to negotiate with anyone, I get time to day dream, read, listen to podcasts on my commute with a coffee. It can be bliss! Equally a wfh day when they're at nursery/school is also super bliss! My home being a quiet sanctuary again - still exists.
So for me it's not 'having children'. It's not the children themselves, it's about what you give up for them. If 24hrs a day, every single day - you get no respite - then I can see how you could become depressed, in much the same way being anyone's career 24/7 would take a natural toll.
Plus it can be so isolating. I really felt that on maternity leave. I love my baby and I was in floods of tears leaving her at nursery to go back to work, but she settled quickly in nursery and I got back into a routine and then that low level depression and feeling of isolation lifted. I look at my colleagues as part of my village. The people who take care of maiden-named me. She still exists.
Now time with my DC is precious. Weekends are a joy. Bedtime is a routine of connection, cuddles and giggles in the morning before getting up are life. If I was at home with them 24 hours a day, I don't think I'd feel that way.