Dear @Melhopenot
A central requirement for naturalisation is that the applicant’s identity has been proven.
The best way in which to do this is via a passport or other identity documents that have a photo. Only if the person cannot provide this, can other documents without a photo (e.g. birth certificate, baptismal certificate) be accepted instead.
However, if a person has a passport and/or other documents with a photo, the authorities may have doubts about whether it is authentic. In this case, they can have the documents examined at the German embassy in the country the documents are from. In such cases, they may also request additional identity documents (such as a birth certificate) in order to better establish a person’s identity.
The question of what is reasonable and unreasonable to require refugees to do in this process is complicated. In German the terms used are ‘zumutbar’ and ‘unzumutbar’. In general, it is seen as reasonable to require a refugee to make contact with friends and family in the home country in order to assist with getting documents, or to pay a lawyer in the home country to do so. In certain cases, contact with the embassy solely for the purpose of getting documents is also considered reasonable. But this depends on the particular country and it is not reasonable if the refugee or their family are as a result put in danger.
You say it is a common practice that others are being asked for a birth certificate. If you wish to obtain German nationality, you should make an application and see if they require additional identity documents to your passport. If they do and you are unable to do so, then you have to explain the reasons with it is unreasonable (‘unzumutbar’) for you to do so.
Here is an article (in English via online translation) about a decision of Germany' Federal Adminstrative Court (the highest court in these matters) on this matter:
https://***-kanzlei--koenigstrasse-de.translate.goog/aktuelles/identitaetsklaerung-bei-einbuergerung?_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp
Best,
Éanna