The problem with depression in particular is that even if you know that things like exercise would help, it is really hard to push yourself to do them. You end up lying in bed feeling miserable and although there is nothing physically stopping you from just getting up and doing stuff, it feels like you are being held down and prevented from doing so and it takes an incredible amount of energy to overcome that.

There are also other reasons people don't take care of themselves, that aren't their fault. So many people these days are overworked and struggling to make ends meet while also juggling other responsibilities like child care and house work. It's totally understandable not to have the energy for exercise and cooking healthy food at the end of the day. In my opinion it's often a failure of the system and less of the individual.

That being said, exercise and taking care of your body absolutely does wonders for mental and physical health.