why

Because it means you get faster access. If each chips can process X number of reads and Y number of writes per second, in your hypothetical 8 chips stick, the controller now can process 8X number of reads and 8Y number of writes per second.

what if I want to get 64 bits that are stored in a single chip

You likely won't. Interleaving means every operation is spread evenly among all chips, and since it's supposed to be transparent (ie, the OS and apps generally won't care if a controller have 2 or 4 or 8 chips), you'd go out of your way to write & read something that end up on a single chip, in the process you'd be writing and reading the rest of the chips anyway. A normal operation of storing or reading something will end up using all chips, very quickly, without you having to care about the details.