It is because, at least in the later borrowings, Semitic ṭ ט is regularly represented by τ [t], while t ת is represented by θ [th]. It has to do with the fact that the Semitic emphatics are aspirated, while the plain stops are not aspirated.

The fricative pronunciation of Greek θ, and of Aramaic/Hebrew post-vocalic t does not emerge until well into the Christian era.

It is because, at least in the later borrowings, Semitic ṭ ט is regularly represented by τ [t], while t ת is represented by θ [th]. It has to do with the fact that the Semitic emphatics are unaspirated, while the plain stops are aspirated.

The fricative pronunciation of Greek θ, and of Aramaic/Hebrew post-vocalic t does not emerge until well into the Christian era.