Have you seen a MicroSD die before your eyes, failing writes, then reads? I did several times.

A RAID array of MicroSDs, or maybe other flash devices, could be more reasonable reliability-wise, but it would be anything but a sleek consumer device.

OTOH, Apple is selling the "experience", the polish and automagic features. Maybe iCloud integration leaves a lot to be desired, it still beats a dongle 99% of the time.

(Disclaimer: Android user since Android 1.x.)