I feel most people got interest in nature by themselves - they went out of the house to the woods, park, pond etc. and became fascinated with wild life there. But this does not work if the child has no forest, park or ponds in the walking distance. An adult without a car quickly finds that forests and wildlife places are worst connected by public transport of all possible points in the country.
I don't know if I would have gotten interested in nature by myself, though no doubt a lot of birders (and others) did. One of my first memories of books is my mother buying the "Ladybird" bird books for me when I was a child (for which I am eternally grateful). Had she not thought that natural history might be something I'd find interesting, I'd most likely be even more of a soulless wageslave than I am now. Mysticete's point about economic circumstances is a good one - I'm fortunate my family, unlike those in many 'marginalized' communities, was able to afford books and to allow me the leisure time to read them.
I, personally, actually prefer urban birding - I'm fascinated by the wildlife that is found in our cities and which coexists with man amidst the human-modified habitat. I'm grateful that forests and other nature reserves exist in the UK and elsewhere, but (speaking only for myself) it isn't necessary to visit such places for me to find wonder and joy in nature.
Personally, I am not convinced that the "spark" is less common than it was when I was growing up. I never ran into anyone interested in nature in the way I was until College...I knew some young hunters and fisherman and that was about as close as I got.
Of course fishing and hunting are also interests in nature. I've spent a lot of time fishing and to succeed in that requires an understanding of your quarry and its environment and the conditions that influence its behaviour that is very similar to birding. I have to say that birding has made me a better observer and record keeper, more attuned to weather, conditions, and activity patterns, all of which have changed my approach towards fishing for the better - not only made me a better angler but increased my appreciation of the sport. Where the two activities come together (for example in tuna fishing, where spotting and understanding the behaviour of birds can be essential) applying the skills learned in one to the other can be very satisfying.
China (the PRC) is interesting in the context of this discussion because it has some of the world's largest urban zones and populations. The PRC is a bit of a paradox in that, while obviously there is a huge amount of wildlife trade that is deletorious for wildlife in both the PRC and internationally, cities like Shanghai do have large city parks that have a good amount of bird life (
see link). We can only hope that with increasing development and prosperity will come increasing appreciation of nature, as has happened in places like Korea and Japan - and Europe for that matter. I've read articles covering the birds of London in the 1940s/50s - it's remarkable, even allowing for (presumably) more observer effort and better observers, how many more species are seen in the same areas today.