There is a great deal of variation in the number of syllables in quantifier phrases. Quantifiers like many, few, all, some, each, every, lots, etc may require an article before the quantifier (a lot) or after a preposition (many men/many of the men/*many of men), may require a preposition (*a lot men) or disallow it (*every of them), or make it optional.
All, in particular, allows an optional of, mostly, I suspect, to give a short word with a big meaning a little more bulk with an extra syllable. So the following noun phrases are all grammatical and equivalent in meaning, but not equally ponderous:
- All three cars ~ All the three cars ~ All three of the cars ~ All of them: those three cars