In Canada, where I grew up, straight men who are friends don't touch each other often. Neither do women for that matter. I never cuddle my friends or family and thus non-essential touching is for me more of a sexual thing. Perhaps this is why I was so.... touched by this subject.
Seeing this behaviour so much in Istanbul, I wondered why my culture was deprived of it.
I discovered that a century ago, men in the West used to pose for photos in very bromantic ways as well. Here is a fascinating article
on the subject full of great portraits. I imagine their day-to-day physical relationships were as affectionate as Turkish men's are today. So... what changed ?
While homosexuality has always existed, in the past it was considered to be more of a behaviour than an identity or permanent sexual orientation. In the first half of the 20th century the public slowly understood that a person could be gay, and they put it in the DSM as a psychological disorder. It wasn't removed until 1973 and the stigma remained for much longer.
I think fear of looking gay stopped men from touching each other in a natural way, but these invisible constraints are finally loosening as homosexuality becomes completely accepted in cosmopolitan societies. The term bromance, coined in the 90's, became a part of popular culture between 2010-2020 when men here and there started acting bromantic again.
For the majority of men in Turkey, as well as in other countries where men have a more physical rapport with their friends (India, the rest of the middle east, the Maghreb....), there is I think still a mentality that it is not fathomable to be gay, so they are not worried about looking so.
Ironically, the more cosmopolitan/gay-friendly men Turkey are actually less
likely to be walking down the street arm in arm.
One other theory about this behaviour is that where men and women are shamed (or worse) for intimacy outside of marriage, they try harder to meet their intimacy needs in friendships. While I think that this might explain some of the actual homosexual activity going on between straight men in Turkey, I don't think this is the reason for these platonic intimate gestures.
My hypothesis is that this is something natural that Western societies have drifted away from for the reasons mentioned above, and that platonic physical intimacy with friends is psychologically healthy.