The function of all, both, and each
Solution 1:
In a comment, John Lawler wrote:
That's because they're Quantifiers, not just pronouns. One thing that all, both, and each can do is appear in their normal position modifying a noun phrase, or appear in an adverbial position before the main verb or (after the first auxiliary verb if there is one). This rule is called "Q-Float". Only some quantifiers can float, however; each and all float, but every and any don't.