Because this expansion is so infinitesimal on that scale as to be negligible. It's about 7%/Gyr, or a billion years for an unbound structure's "space" to grow by 7%. Or about 70 picometers (10^-12) per meter every year. Or in other words, about a water molecule per meter every four years.

At that rate nearly every other force (from chemical bonds to gravity) prevent anything but the largest cosmological structures (like galaxies) from being effected. They simply continue with their system as they always have, and the extra space shows up as distance between galaxies.