Why do we (mothers) always get the blame even if we're not available or very busy?

I really enjoyed motherhood even though I've been tired some days. Though we're expected to be nearly perfect as mothers and make no mistake. Even if we make a trivial mistake where no one is hurt, just something silly; we get harshly reprimanded. We always get the blame if anything happens to our child, even if we're not available at that moment or busy and our child is with another caregiver we trust.

Here are a couple real examples that the mother still got the blame when she wasn't even available:

- A 2 year-old girl fell to her death from the 11th floor under her father's care. The girl's mother was obeying the court order, which dictated that the father has vistation rights on Sundays. The man fell asleep from a hungover and is now facing negligent charges. Yet several people on FB comments still place the mother at 50% blame instead of 100% blame on the father.

- A father accidentally dropped her 7 month-old baby girl while helping his elderly mother at a mall. The article clearly stated the baby fell from her father's arms and several people on FB and Youtube still ask where was the mother. The mother was in the bathroom and was going to be right back.

I have my own example too:

When my now 12 month-old son was 7 months old, my husband was cleaning a spit up with a tower and within only one second of taking his eyes off, our son rolled off the bed and fell on the hardwood flooring. I was busy making our son's breakfast while my husband was taking care of him. I couldn't reach him on time because it happened right there as soon as I returned to the room. My husband actually took the blame. He said it himself that it was his fault for taking his eyes off for even a second. Luckily nothing happened. He's now a very active baby that will walk soon.

My mother's logic: ''But where were you, couldn't you grab him on time''? No, I was making breakfast. How if I was still far from reach and it happened right there when I came back?

Updates
2 mo
If I'm not there at that moment (ex: I'm out buying food and formula for my baby) and the son is under my husband or parents' care then they would be the ones responsible at that moment.
Why do we (mothers) always get the blame even if we're not available or very busy?
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