Good question. I don't really know. Maybe some men have no hope of finding a woman in the conventional way, so they do this. Women may believe that they have more hope in finding a man.
with specific regard to heterosexual marriage, the gender ratio for 'success' is surely 1:1, no?
Most likely
What I'm saying is that for example, on a night out, it would be much easier to pick up a girl than the other way around, so girls don't need to worry about not being in relationships as much.
the mail order bride industry seems to be huge and growing every year yet the other way around it doesn't even seem to exist. it's really weird
There are, and many. There are whole forums on middle aged women, who brought for themselves grooms from Egypt, Morocco, Middle east, mostly engineers. There engineers then pass an exam to become an engineer in the US, the UK, and these women have men of them same age, who are good providers. There is a huge demand for them. You maybe meant that there were no specific web sites with such grooms, like, handsome young Egyptians, all engineers, click here to join. Mostly these women get these men off general dating site. I think that there are no sites like this because there are still double standards: if a man gets himself a younger mail order bride, he is "a lucky dog", if a woman does it, she is "desperate". Also, there is a huge industry of toy boys and professional gigolos in places like Turkey, Morocco, Cyprus (seaside areas). Many of these gigolos become mail order grooms in due course.