I disagree with the first two factors, as a European. I live in a city where people walk and cycle a lot, but that does nothing for loneliness. No, seeing random strangers on the street is not a substitute for a meaningful relationships, and no, these random strangers you see do not make friends either.
Factor three is probably a major one in all countries where people are spending time staring at screens instead of interacting with each other.
Factor #4 doesn't really show up much in my corner of Europe.
I think this is where US culture and European culture differs a lot. For some reason, in US everyone needs to interact with anyone who stays longer than two seconds in close enough proximity. I am so thankful this is not a case in Europe (at least where I am) — for an introvert hating chit-chat it gets annoying really fast.
“ Proximity, unplanned encounters and the privacy to confide with someone have been identified as key factors in making friends as an adult.[cite]”
Factors that make it difficult for Americans to make friends as they age:
1. Isolation driven by car culture.
2. Un-walkable communities outside cities.
3. Media driven self absorption vitiating real world interaction.
4. Political and social siloing as a result of #3.
“Bowling Alone” by Putnam was prescient by did not offer any realistic solutions.