It's not that way because somebody in the 70's flipped a coin and decided that sync was heads.
It's because it's a reasonable assumption to make. Data loss shouldn't be a surprise, if I need speed and am willing to risk dataloss I should have the option, but should explicitly choose to use it.
> if I need speed and am willing to risk dataloss I should have the option, but should explicitly choose to use it.
You did, by choosing to use MongoDB.
(And if you chose MongoDB without being aware of that implication, you didn't choose MongoDB for the right reasons or didn't do your due diligence, because you cannot understand MongoDB's use case and tradeoffs if you were unaware of this.)
