Why not? ¯_(ツ)_/¯
The accent moves a lot actually, sometimes because as the form changes the word might get extra syllables and the accent can only be in one of the last three, so you need to move it further down. It must also have something to do with the original short and long vowels in Ancient Greek and the three-syllable rule and how all these things come together. Why is ambiguous pronounced as /amˈbɪɡjʊəs/ while ambiguity is /ambɪˈɡjuːɪti/ and not /amˈbɪɡjuːɪti/ ?