Why do textbooks view 自発 and 受身 as two distinct forms?
[受身]: 誰かがりんごを食べる --> 誰かにりんごが食べられた
[自発]: 彼女が両親のことを思い出す --> 彼女に両親のことが思い出される
Grammatically, both forms take the noun marked by を, and mark it by が (at least in the simplest case).
Semantically, the two forms are also quite similar, both emphasizing some loss of control. My apple was eaten by someone, without my permission. The girl's memories came about suddenly, taking away whatever little control she had with 思い出す. (Admittedly, the person who loses control is different in these cases: with 受身, they weren't mentioned in the original sentence; with 自発, they were marked with が in the original sentence.)
Am I missing some fundamental difference between these forms?