Not a native speaker, but I speak Mandarin.

I'd say it's because to a Mandarin speaker A.P.P is easier to pronounce than 'app' (Mandarin doesn't have any syllables that end with a p sound, but it does have syllables that start with p).

There's also a history of 3 letter acronyms imported from English, such as DVD, VCD, MP3 which are all pronounced letter by letter and even though APP is not an acronym, to a native speaker it fits a similar pattern.

I'm a native mandarin speaker from Taiwan and now living in NY. I think it just comes down to the fact that english is not everyone's first language there.

For your example, people end up pronounce A.P.P because not everyone knows that app comes from `application` and therefore you might run into conversations with people speaking with different pronunciations. Some people pronounce 'app' the normal way, and some people pronounce 'app' similar to (ape).

After awhile, people would explain what they're trying to say by spelling out, and eventually you'd learn to just say A.P.P instead of attempting to incorrectly pronounce the word.

Same thing goes with Facebook, which is referred as F.B, and Instagram, which is usually referred as I.G

Instagram is "eye jü". Nobody in Taiwan really says G.

It's because Mandarin has standard syllables. While the sound for "p" is in the language, it's normally bound into a syllable. So saying "app" with the "p" sound at the end is awkward, but "pi" is natural.

IIRC, Whatsapp is the first popular app of smartphone in Taiwan. In the beginning, A.P.P stand for Whatsapp in TV talk shows, just like F.B(Facebook) & I.G(Instagram) nowadays. Hence people trend to spell the word app as A.P.P.. As more and more apps came out, the word app became more general, but the spelling didn't change at all.

Funny, just recently I had to explain this to my Taiwanese friend. I didn't know what the heck he was talking about when he said his colleague was working on A.P.P. thought it was an agency or something.