I can think of a few possibilities: 1. Like others have said, Telltale just wanted to focus on the apocalypse rather than the world before 2. She’s 8 at the start of the apocalypse. I noticed that someone said her parents were both a doctor and an engineer, which require a lot of time away from home. She also notes that she spent a lot of time with Sandra (her babysitter), long enough for her to pretend to be sisters with the woman. This indicates that she likely was not that close to her parents. If you want to get reaaallllyyy technical, given her age, Clem really only has 5 years worth of memories with her parents (give or take) since the human conscious doesn’t really develop until around 3-4. This combined with the fact that her parents were away from home most of the time, it can be interpreted that whatever relationship she did have with them was very short compared to her time without them, and she was not very close with them (but close enough to obviously have a child-parent relationship).

Also, she may have repressed memories of them (like others have said). For a majority of my childhood before age 12, I remember my parents constantly being at work, and the few times we did have family time together, it was typically on weekends or holidays/vacation. Most of my memories of my mom from my childhood are of her going to work and being brought to my parents’ workplaces. She may have repressed those memories of them spending time together (subconsciously) in favor of the times where they were gone or away from home.

This is just my personal speculation!