The final e only gets an acento circumflexo and is therefore pronounced as a closed e, see 3.4.3, if it is stressed and in opposite to the Spanish interrotagive adverb por qué this is not the case with the Portuguese porque (European Portuguese) / por que (Brasilian Portuguese). However if por que comes at the end of the sentence, "The question is why?", than the final e is stressed an then it gets an acento circunflexo. However the distinction made depending on the position in the sentence and the function of por que are perhaps a little bit to complicated, and therefore we can find any amount of examples where por que before of a question mark and a point, in other words at the end of a sentence, has no accento circumflexo. (What is by the way a rule easy to remember. Before a question mark or a point, put an acento circunflexo on the o.) Needless to say, that this is an irrelevant aspect for the learner of Portuguese. No need to know complicated rules that only concerns the writing that not even native speakers use correctly.
A little bit subtle is the difference between por que (why) and porque (because)
I know why he did it. => Eu sei por que o fez.
I know it because he did it. => Eu o sei porque o fez.
|por que (written in two words in euorpean protuguese)
|interrogative pronoun why
|Por que você não vai ao cinema?
|Why you don't go to the cinema?
|causal conjuntion why
|Não sei por que não quero ir.
|I don't know why I don't want to go.
|why (noun: actuall a shortened full sentence)
|why if put at the end of a sentence
|Ela não me telefonou nem me disse por quê.
|She didn't call me and she didn't tell me why.
(She didn't call me and sheh didn't tell me why she didn't call me.)
|relative adverb (by which)
|by / through etc. + which / what
|Os lugares por que passamos eram encantadores.
|The places we passed by were very charming.
|causas conjunction because
|Não fui ao cinema porque tenho que estudar para a prova.
|I didn't go to the cinema because I had to learn for the exam.
|Why as a noun with article. The englisch translation is with "the reason".
|Ninguém sabe o porquê de tudo isso.
|Nobody knows the reason of all that.
|Interrogative adverb at the end of a sentence. (Written in one word in European Portuguese.) Actually it is a shortened question.
|Andar cinco quilômetros, por quê? Vamos de carro.
|To walk five kilometers, why? Let's go by car.
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