In Greek the consonant cluster ct (or kt) is spelled “KT” or “κτ” (kappa tau). In Latin, this cluster was transliterated as “CT”. English took the spelling of Greek words, including proper names, from Latin, so that the name Κτιμινη and Κτησιφων are spelled Ctimine and Ctesiphon, respectively, in Latin and English.
Every language has constraints on the way its sounds can be organized in words (“phonotactics”).
https://en.wikipedia.org/wiki/Phonotactics
In English, there is a phonotactic rule that prohibits the consonant cluster ct- or kt- (it could be spelled either way in English) from occurring at the beginning of a word. As a result, the combination of consonants spelled ct- in English at the beginning of a word of Greek origin is often reduced to simply t- in English pronunciation. (But some of us, myself included, pronounce the cluster with a k sound.) Of course, the cluster -kt- in other positions i.e., when it doesn’t occur at the beginning of a word, is perfectly pronounceable in English: “act”, “acting”.
Similar constraints of English phonotactics result in:
–a reduction the Greek letter ψ-, which represents the consonant cluster ps-, when it occurs at the beginning of a word, to the single consonant s- in English words derived from Greek, e.g., “psychology”;
–a reduction of the Greek letter ξ-, which represents the consonant cluster ks-, when it occurs at the beginning of a word, to the single consonant z- in English words derived from Greek, e.g., “xenophobia”, “Xenophon”;
–a reduction of the Greek consonant cluster πν- (pn-), when it occurs a the beginning of a word, to the single consonant n- in English words derived from Greek, e.g., “pneumatic”; and
–a reduction of the Greek cluster consonant cluster μν- (mn-), when it occurs at the beginning of a word, to the single consonant n- in English words derived from Greek, e.g., “mnemonic”.
The Greek consonant cluster κν- (kn- or cn-) is another consonant cluster that can’t occur at the beginning of an English word, but the only Greek-derived English words I’ve been able to find beginning with the letters cn- are “cnidaria” and “cnidocyte” (and maybe some derivatives of these), and I’ve never heard them pronounced.
https://en.wikipedia.org/wiki/Cnidaria
https://en.wikipedia.org/wiki/Cnidocyte