For the same reason why in English 12 = twelve, but 14 = fourteen. Not all numerals are formed by the same rule.

The correspondents of the -teen numbers in Greek were formed similarly with English, after the model of 14 = four and ten = "tetrakaideka".

"Tetrakaideka" is a contracted form of 14, normally used in compound words. When "14" was an isolated word, it would have been "tettarakaideka" or "tessarakaideka". These are the forms for the neuter gender, the numeral "4" = "tettara" or "tessara" (depending on the dialect) was changed by declension for other genders and cases.

In Ancient Greek numbers bigger than 20, the word "and" = "kai" was usually omitted, but then the bigger number was always the first like in "twenty-four". When "and" was inserted, then the order could also be inverse, like in "four-and-twenty".