Because it's modifying the noun helyek. In this position, an adjective doesn't take any endings. You can think of csendes helyek as one unit; the unit only needs the plural ending once, and it goes on the noun.

On the other hand, if you just wanted to say "Attics are quiet" then it would be

A padlások csendesek.

(It's as if, since there's no noun to take the plural, the adjective is stuck with taking it.)