It usually means the parents did not bond well with them during their formative years or there was something that took place which forced a wedge between them.
I grew up in a household where my mother was overbearing, abrasive, prone to yelling and whipping me and my brothers during our childhood. Naturally that created some distance between us when I went away to college and eventually moved 2000 miles away.
Childhood was something I wanted to escape from. I wanted to be in control of my life and making my own decisions early on.
However over time we got closer and spent some quality time together in years prior to her death as we visited with one another.
I wouldn't hold it against anyone who was not close to their parents.
Life is a (personal) journey.
Each of us gets to (choose) who we want to spend our time with.
Sometimes the most toxic people in one's life are their parents. I don't believe anyone should feel "obligated" to associate with people that poison their lives or cause them unhappiness even if they are blood related. No one deserves a "free pass" to make you miserable.