Can only speak to my own experience but mainly, it was having kids. I went from working four nights a week with plenty of time to eat properly, exercise plenty and ensure my mental health was doing okay.

Suddenly I've got two young kids (5 & 2) and I've had to switch to a 45hr a week day job that's got significantly less walking than my previous one. I don't see my partner (she works when I'm on my weekends) and so we sort of just eat whenever the kids go to sleep, which usually means we eat poorly, or a takeaway. And on top of that my mental health is in the toilet so I find it really hard to even want to take care of myself at times. Why spend an hour of my very limited free time going for a run or cooking a healthy meal when I could sit down, order food and play videogames before going to bed to begin another day if bullshit.