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" soun

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.