I assume they evolved to have the inedible husk to prevent damage while being eaten and increase the distance from the original tree, effectively delaying germination. (If you can describe evolution as having an intention…)
Nature is pretty clever, imho.
These plants usually evolve to serve a particular ecological niche. It's kinda like the soil's immune system. Exposing bare soil to direct sunlight is one of the most harmful things you can do to a soil ecosystem. So many plants evolved seeds that lay dormant until they're needed to come and protect the soil
Plants like dandelions are known as major weeds, but they only germinate in soils that are too compacted (e.g. a farmland with a history of plowing). These dandelions take over, decompact the soil, and then get "outcompeted" by other plants (usually perennials with longer lifespans).
I use scarequotes for competition because it's a good example of a natural system where "competition" is a lens of analyzing nature that could cause you to miss the bigger picture
Relevant xkcd: https://xkcd.com/1259/
The megafaunal partner of the mango is probably the elephant, which is still extant in the native range of the mango.