While all of these are valid arguments, no one said restaurants are always clean and grandma's house is always dirty. But if we are going to make generalized arguments about what MIGHT be going on in a particular place, we can make the same arguments and assumptions about what MIGHT be going on in grandma's kitchen.
So if we apply the same over generalized assumptions, and someone says it doesn't make sense to eat in a restaurant, but not at grandma's house... then my only point is that theoretically training is required in restaurant kitchens but not for grandma. I'm just applying a little logic to why they might make that recommendation, not intending to impune anyone's grandma, and certainly not suggesting that every commercial kitchen is exactly what it should be. On a level playing field though, training and knowledge favors the professionals.
On the flip side, most people probably have some experience with their own grandma and her hygiene level, as well as her level of care and concern over her family's health and safety.
Another reason when my mom drops food off at my house, it usually ends up in the trash.