As per smritis only people born to parents who are both Brahmin parents can be Brahmins. For example, in Manusmriti 10.5, 10.10 and 10.14 it says:
सर्ववर्णेषु तुल्यासु पत्नीष्वक्षतयोनिषु । आनुलोम्येन सम्भूता जात्या ज्ञेयास्त एव ते ॥ or Among all castes, those ONLY who are born of consorts wedded in the natural order, as virgins of equal status, are to be regarded as the same (as their father)
विप्रस्य त्रिषु वर्णेषु नृपतेर्वर्णयोर्द्वयोः । वैश्यस्य वर्णे चैकस्मिन् षडेतेऽपसदाः स्मृताः ॥ or "Children of the Brāhmaṇa from the three lower castes, of the Kṣatriya from the two lower castes, and of the Vaiśya from the one lower caste,—these six have been declared to be ‘LOWBORN’"
पुत्रा येऽनन्तरस्त्रीजाः क्रमेणोक्ता द्विजन्मनाम् । ताननन्तरनाम्नस्तु मातृदोषात् प्रचक्षते ॥ or The sons of twice-born men from women of the next lower caste, who have been enunciated in due order, are CALLED BY THE NAME of the lower caste, on account of the taint attaching to their mothers.
Vedas were revealed to a variety of people like Jabala (unknown father, shudra mother), Vatsa (shudra mother), Vishwamitra (kshatriya parents), Ailusa (shudra mother) etc. These people were all Brahmins, but would not be as per smritis.
Agamas/Tantras are directly revealed, sometimes to people not born by Brahmins.
Puranas & Mahabharata are by Vyasa, a mixed-caste, who was born to varnasankara or mixed-caste mother - Satyavati.
So why even one type of scripture, except Ramayana, is not written by people born to both Brahmin varna parents? Does this mean smritis are not accurate or interpolated?
(NOTE: Caste/Smriti supporters or apologists please don't downvote or close this question. If you have problem, please say it in comments)