How about because there are no original Hebrew or Aramaic original texts...No evidence that there ever was...
All of the "Church Fathers", both East and West, testified to the Semitic origin of at least the Book of Matthew, as the following quotes demonstrate:
Papias (150-170 C.E.) Matthew composed the words in the Hebrew dialect, and each translated as he was able. (quoted by Eusebius Eccl. Hist. 3:39)
Ireneus (170 C.E.) Matthew also issued a written Gospel among the Hebrews in their own dialect. (Irenaeus; Against Heresies 3:1)
Origen (c. 210 C.E.) The first [Gospel] is written according to Matthew, the same that was once a tax collector, but afterwards an emissary of Yeshua the Messiah, who having published it for the Jewish believers, wrote it in Hebrew. (quoted by Eusebius; Eccl. Hist. 6:25)
Eusebius (c. 315 C.E.) Matthew also, having first proclaimed the Gospel in Hebrew, when on the point of going also to the other nations, committed it to writing in his native tongue, and thus supplied the want of his presence to them by his writings. (Eusebius; Eccl. Hist. 3:24)
Pantaenus... penetrated as far as India, where it is reported that he found the Gospel according to Matthew, which had been delivered before his arrival to some who had the knowledge of Messiah, to whom Bartholomew, one of the emissaries, as it is said, had proclaimed, and left them the writing of Matthew in Hebrew letters.
(Eusebius; Eccl. Hist. 5:10)
Epiphanius (370 C.E.) They [the Nazarenes] have the Gospel according to Matthew quite complete in Hebrew, for this Gospel is certainly still preserved among them as it was first written, in Hebrew letters. (Epiphanius; Panarion 29:9:4)
Jerome (382 C.E.) “Matthew, who is also Levi, and from a tax collector came to be an emissary first of all evangelists composed a Gospel of Messiah in Judea in the Hebrew language and letters, for the benefit of those of the circumcision who had believed, who translated it into Greek is not sufficiently ascertained. Furthermore, the Hebrew itself is preserved to this day in the library at Caesarea, which the martyr Pamphilus so diligently collected. I also was allowed by the Nazarenes who use this volume in the Syrian city of Borea to copy it. In which is to be remarked that, wherever the evangelist... makes use of the testimonies
of the Old Scripture, he does not follow the authority of the seventy translators [the Greek Septuagint], but that of the Hebrew." (Lives of Illustrious Men 3)
"Pantaenus found that Bartholomew, one of the twelve emissaries, had there [India] preached the advent of our Lord Yeshua the Messiah according to the Gospel of Matthew, which was written in Hebrew letters, and which, on returning to Alexandria, he brought with him." (De Vir. 3:36)
Isho'dad (850 C.E.) His [Matthew's] book was in existence in Caesarea of Palestine, and everyone acknowledges that he wrote it with his hands in Hebrew…(Isho'dad Commentary on the Gospels)
Other "church fathers" have testified to the Semitic origin of at least one of Paul's epistles. These "church fathers" claim that Paul's Epistle to the Hebrews was translated into Greek from a Hebrew original, as the following quotes demonstrate:
Clement of Alexandria (150 - 212 C.E.) In the work called Hypotyposes, to sum up the matter briefly he [Clement of Alexandria] has given us abridged accounts of all the canonical Scriptures,... the Epistle to the Hebrews he asserts was written by Paul, to the Hebrews, in the Hebrew tongue; but that it was carefully translated
by Luke, and published among the Greeks. (Clement of Alexandria; Hypotyposes; referred to by Eusebius in Eccl. Hist. 6:14:2)
Eusebius (315 C.E.) For as Paul had addressed the Hebrews in the language of his country; some say that the evangelist Luke, others that Clement, translated the epistle. (Eusebius; Eccl. Hist. 3:38:2-3)
Jerome (382) “He (Paul) being a Hebrew wrote in Hebrew, that is, his own tongue and most fluently while things which were eloquently written in Hebrew were more eloquently turned into Greek (Lives of Illustrious Men, Book 5)
It should be noted that these church fathers did not always agree that the other books of the New Testament were written in Hebrew. Epiphanius for example, believed "that only Matthew put the setting forth of the preaching of the Gospel into the New Testament in the Hebrew language and letters." (Epiphanius; Pan. 30:3) Epiphanius does, however, tell us that the Jewish believers would disagree with him, and point out the existence of Hebrew copies of John and Acts in a "Gaza" or "treasury" [Genizah?] in Tiberius, Israel. (Epipnanius; Pan. 30:3, 6) Epiphanius believed these versions to be mere "translations" (Epiphanius; Pan. 30:3, 6, 12) but admitted that the Jewish believers would disagree with him. The truth in this matter is clear, if Greek had replaced Hebrew as the language of Jews as early as the first century, then why would fourth century Jews have any need for Hebrew translations. The very existence of Hebrew manuscripts of these books in fourth century Israel testifies to their originality, not to mention the fact that the Jewish believers regarded them as authentic.
In addition to the statements made by the early Christian church fathers, the ancient Jewish Rabbis also hint of a Hebrew original for the Gospels. Both the Jerusalem and Babylonian Talmuds and the Tosefta relate a debate among Rabbinic Jews over the method of destruction of manuscripts of New Testament books (t.Shab. 13:5; b.Shab. 116a; j.Shab. 15c) . Specifically mentioned is a book called by them as ALEF-VAV-NUN-GIMEL-LAMED-YUD-VAV-NUN (see end note) (or "Gospels"). The question which arose was how to handle the destruction of these manuscripts since they contained the actual name of God. It is of course, well known that the Greek New Testament manuscripts do not contain the Name but use the Greek titles "God" and "Lord" as substitutes. This is because the Name is not traditionally translated into other languages, but instead is (unfortunately) translated "Lord", just as we have it in most English Bibles today, and just as we find in our late manuscripts of the Septuagint. The manuscripts these Rabbi's were discussing must have represented the original Hebrew text from which the Greek was translated.