In countries/cultures where it's customary to eat dinner very late in the evening (10 pm), why aren't there ill effects as there would be in

I don't know for sure, but one reason may be that, in those countries, the largest meal of the day would be the midday meal so that dinner would be more like our lunch.

More than a year ago i began doing a late supper around 10 pm or later just before bedtime usually oatmeal soaked in milk for about an hour with stewed prunes and some cheese or meat following.  For a couple of weeks I was having trouble getting used to this but works ok now. 
Reason for late supper was because I regularly go out around 5 pm and get home 9 pm or later. I have a late lunch so then a late supper is OK. 

Generations of people can become accustomed to something. There are entire tribes where the lactose intolerance rate is almost 100%. Yet it's rare in Western nations, which seems to suggest that lactose intolerance is a "default" that can be overridden.

That said, if I eat that late I always get heartburn, no matter what it is that I eat. 

not sure but my parens would wait till my father came home from work and we'd eat at 9:30 at night most of the time

I have some cousins who live in Argentina which is one of those countries that has dinner very late in the evening but they have five smaller meals throughout the day but the biggest meal of the day is usually lunch which they have in early afternoon.