When you think about it, "from" can sort of work in this context. I remember to use "el" by thinking it is made "from something", which is not proper English in every context, but it helps me out.
But remember, words often don't have one direct translation, especially prepositions! It may help to not directly associate "el" with "from" so strongly.
And this feature is not unique to Esperanto! In German the same rule applies, so you translate "es ist aus Silber" to "it is made out of silver". "Aus" can mean from, and it does in the context of saying where you're from (e.g., "Ich komme aus Deutschland") just as "el" does for Esperanto.
Perhaps things work similarly in other languages too, I wouldn't know XD
I hope this helps, and if anything I have said here is wrong, please correct me! Mi estas nur komencanto :)
It is because Esperanto grammar is strongly influenced by Russian language. "Unua libro" was published mainly in Russian, and it was also the most used by Zamenhof language. "El plasto" is almost the same as "из пластика", and if you want to say that someone is from another country you can use "из другой страны".