Probably these too, but if I compare mu childhood (sixties) and society today, the experience kids got/have are in especially sharp contrast. When I was young I was often dumped into large family gatherings which lasted days (birthdays of (grand)grandparents, funerals, weddings etc). I had to practice handling cousins etc who might had very different family backgrounds than me since very early age. We had to find things we had in common and accept our differences. We learned that differences are manageable.

It's not common nowadays. Many people don't have relationships with relatives at all and kids don't meet another kids with different background until school. And even then distance is kept often because of overprotective parenting. If I look at my students (highschool and college level), most of them are absolutely terrified to interact with people very different than they themselves. A single difference is enough to keep distance, dump relationship at all. They are not used to it at all.

Speaking for myself there is one other factor that plays a huge part:

4: Nearly every time a stranger tries to talk to me it is to beg for money or sell me something (which is also begging)

In fact I'd say this is by far the prime reason I don't interact with people I don't know. I'm not a kid, however.