Why do we distinguish the spontaneous (自発) and passive (受身) forms?

Question

Why do textbooks view 自発 and 受身 as two distinct forms?

Example:

[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?

Answer 1

自発 and 受身 are two of the meanings of the same form. Syntactically, they are the same. Semantically, they are related but different. Japanese textbooks for Japanese students treat the two (ら)れる as the same 助動詞. For example, please take a look at this page.

_{(Compare must in English; must as in "You must be brave" and must as in "You must be hungry" are syntactically the same but semantically different.)}

A typical 自発 sentence looks like this:

エンジンが故障していると思われます。

If we took this 思われる as passive, we would get something like "It is thought that the engine is broken", but that's not how we normally translate this sentence into English. This use of 思われる indicates the idea came out of nowhere (spontaneously, "outside of control"), and it's basically a milder version of 思います. So it is more natural to translate this without a passive form, like "I got an impression that the engine is broken".

That said, the two meanings are etymologically the same and indeed based on the same meaning, "without one's will". See: Is there a reason why the passive and the potential form are identical (at least for える/いる verbs)? I think it is enough if you understand English passive forms are not necessarily used when translating a sentence like this.