Its unclear if you are talking about yourself or making a general observation about people around you, but in case its the latter: I believe this to be very much subject to strong confirmation bias. You dont open up because you believe thats just not what people do. But I find that to be false 9/10 times I break through the fear and show genuine interest.
Safe spaces would include anywhere people that share a common interest gather, for me thats a climbing gym I go to regularly. Volunteering is also great in regards to meeting people that share similar values.
The issue with being friends is that it mostly works if it's actually present continuous, not present perfect.
That is, to be friends means to be able to find time and interesting stuff for each other with some regularity, on a scale of weeks, not years. Adult life often leaves little time for that, especially now that various forms of easy-to-reach entertainment compete with friendships. Another obvious bottleneck is having (young) kids. To open up is actually a non-zero effort and a noticeable time expenditure; plus you have to allocate time to listen to your friend, too :) Without regular nurture a friendship withers. (Ask me how I know.)
Kids just have so much more time in their hands. (Unless they are also overloaded like tiny adults, then they also have trouble finding friends and, importantly, hanging out with them.)
This is all true but at the end of the day its on you to make that time. If you try and make it happen, people will try and make themselves available or try and reschedule. Especially if they have kids, they are probably chomping at the bit for excuses to hang out with adults.
Safe spaces would include anywhere people that share a common interest gather, for me thats a climbing gym I go to regularly. Volunteering is also great in regards to meeting people that share similar values.