I'm asking this question because I think it is crazy that the only people who ever got the EMS in Naruto series are reincarnations of Indra.
Does someone need to be a reincarnation of Indra to be able to get the EMS?
Although it's quite weird that only Indra reincarnations are the only ones that have achieved Eternal Mangekyou Sharingan, it does not necessarily mean they're the only ones who can awaken it.
According to https://fear-world.fandom.com:
The Eternal Mangekyo Sharingan is the second strongest dojutsu kekkei genkai of the Uchiha clan, being a more powerful variant of the Mangekyo Sharingan. this dojutsu can only be awakened by a user of the standard Mangekyo Sharingan transplanting the eyes of a relative who has also awakened that dojutsu.
Based off of the info given in the website, in order to achieve an Eternal Mangekyou Sharingan, one must awaken a Mangekyou Sharingan, take the eyes of a close relative (usually brother or sister) who also possesses the Mangekyou Sharingan, and thus an eternal one awakens.
The only shown to awaken this are Sasuke Uchiha and Madara Uchiha.
No, you do not need to be a reincarnation of Indra to get EMS (Eternal Mangekyo Sharingan). To obtain EMS you must take the eyes of another Mangekyo Sharingan user, but they must be family, most commonly a brother. Examples would be Sasuke taking Itachi's Mangekyo Sharingan, or Madara taking Izuna's Mangekyo Sharingan. You have been confused because Madara and Sasuke are the only people in the Naruto-verse to have EMS, and they are reincarnates. Itachi's "Goal" was to obtain EMS from taking Sasuke's eyes (He is not a reincarnate).