How can some people, even though they had an amazing friendship or no fallout just stop talking to each other?

I know my dad told us that sometimes even best friends grow apart and don’t even see each other again, but some stay friends. Some people that you hung out with the high school or some people that been really great friends just goes their own separate ways and unfortunately that’s life.
That sounds really sad. I have two best friends that I’ve known since I was seven and there’s no way that we’re letting that happen. We’ve been best friends for almost 12 years and we’re gonna be friends forever.
My sister said that she still best friends with her best friends since they were like six. They don’t talk to each other as much as they used to but they talk to each other a couple times and only hang out a few times a year. She lives in Boston now. I don’t know what my brothers best friend since I don’t really see them much. My dad said that he hasn’t seen his best friend in like 40 years. There’s people I’ve known from high school but I haven’t spoken to them much.
You know I might try to keep everyone together I hope I can so we can do the things we used to do but everyone keeps going separate. Look up pictures and it’s so sad does not like that anymore but we remember the great times.