Why are truncated MP4 files unplayable instead of partially playable?

While the intact part of a truncated webm files (VP8 and VP9) and truncated AVCHD footage is playable, truncated MP4 files are completely unplayable.

More information about it here.

I am not asking here how to repair it, but the technical reason for why it is unplayable.

And why was MP4 not designed to be playable when truncated?

I’ve seen MP4 files that were playable when truncated, but I think it depends on the encoder or its settings.
But, yeah, by and large, mp4 files are not resilient.