Some things accumulate over your lifetime. You don't need to make the same experience over and over again if you can remember it instead. Also energy goes low for many, so they meet up once per month instead of several times per week, just to save energy and regenerate. Life doesn't really get worse because of it. Time just moves faster and faster.

I would reread the chart. Time with friends and time with partner stays the same through 60. You just don’t work as much, and your kids move out. It doesn’t seem so tragic to me.

I'd say the only major thing is time spent with friends sharply decreasing, but that happens in the 20s. Family is an odd one since there is also the category of children and partners. I guess family means your parents, siblings, and extended family? In general speech I would say spending time with your partner or kids counts as spending time with your family.

I also don't think the difference is that drastic over time. The charts shows people spending about 4-5 hours a day alone in their 30s-40s, and then 7-8 hours alone when they get into their 70s-80s. 2-3 hours extra per day doesn't seem like that much to me, especially when you consider the main reason is retiring from work