Sincerity is vulnerable and teens already feel plenty vulnerable.
Also, if you drop the act around your parents, you are acknowledging that it’s an act. Some kids are okay with that because they’re aware of social masking and code switching, consciously or otherwise. Other kids, especially kids who might be struggling socially or are trying to “discover” themselves, would resist the idea that their more popular, more well-liked self is just an “act” and feel pressured to keep it up 24/7, because they assume all the other “cool” kids are naturally like that, too.
If kids are still trying to maintain an aloof/”cool” act around people like their parents then it might be a sign they lack confidence in their social skills and/or have social anxiety.
Also depending on their age, kids are trying on personalities like trying on pants. I remember cycling through some forced personality traits trying to “discover” myself as a tween. Their parents might seem like a “safe” target to crash-test their new, edgy personality on, even if it sucks for the parents.