Why is the iyor tense not being used
While the tense used is correct for a more general statement, surely the iyor tense is more usual and becoming more dominant, isn't it?
So, for "I eat an apple" wouldn't I use, "elma yiyorum" rather than "elma yerim" (I've raised the issue of overuse of pronouns in another post)?
On Duolingo, you mean? Probably to teach the aorist as well. "I eat an apple" can perfectly be translated with the -iyor. I dare say it's more correct, even, to do so.
Aorist is a pain in the ass to explain. There is a common tendancy to make the equation: aorist = simple present, but that's only valid for maybe the 10% of the cases. For the rest, -iyor would be preferred. One simply cannot translate "I live in Istanbul" as İstanbul'da yaşarım. That's just plain wrong.
My advice, if I have any, is... first of all, learners will all come to their own conclusions on this. It's such a vague topic... I would recommend a learner always opt for -iyor to translate a simple present tense in a simple sentence, and reserve the aorist for if-clauses and combinations such as: aorist+ken etc.
As well as what Ektoraskan has said, the present continuous tense (-iyor) is the next tense taught on the course tree, right after the second checkpoint (16th skill). For Duolingo, I think this is much earlier than other tenses tend to get taught in the other languages, probably because of its importance for the reason you have mentioned. (For example, Swedish doesn't teach another tense until skill 30, Spanish not until 29, Irish 38 - all of which are past tenses.) This perhaps does not fully answer your question as to why the iyor tense isn't taught from the start, but I hope it shows that the course creators do in fact consider it quite important.
Actually, things are a lot more different than what you say;
Elma yiyorum = I'm eating an apple [present continuous tense] Elma yerim = I eat an apple [simple present tense]
The first one means that I'm eating an apple just now, yet the second one means you eat an apple, no matter the time. Not any one of them is becoming more dominant over the other.
Still, it's good to remember that some people, like Cypriot Turks never ever use -iyor. They always talk with simple present tense.