I'll no doubt be castigated by some for saying too much, but after my husband died it was a long time before I could force myself to leave the house. (This was no doubt exacerbated by the fact that it happened during lockdown.)
The start of my getting back to a semblance of normality was when I ventured out to a cafe where we had been regulars. DH's stroke meant that only certain places were accessible to us.
Round our way, many of the eateries only serve food at specific times. If you want to eat out because you really can't bring yourself to cook for one, then you have no option but to go out at the busier times. (Go later and it might be open, but it'll be coffee and cake only.)
When you've been your spouse's carer for years and they die, the reason for your existence has been taken away from you. Not everyone copes the same way, but I found that when I went out to get a bite to eat the very act of having something to eat and drink gave me something to do when I simply did not know what to do with myself or how to cope with being outside after shutting myself away for so long.
I could go to the cafe, exchange pleasantries with the staff, look out the window...You'd see other widows and widowers there. You could usually tell. You might not talk, but you'd at least nod to one another.
Very few people choose to finish up on their own. Sometimes it happens. You either try to make your way back out into the world or you curl up and die.
As for mums going out with babies - sometimes they're on their own as a parent. Sometimes, they have a partner but just need to get out during the day for the sake of their sanity.