Actually, /64 is the smallest indivisible network. You can divide that further, but then you lose this network/host identifier split and IPv6 features that require that (stateless autoconfiguration, privacy extensions...). So in case of normal network, ISP handing out /64 restricts topology of you network in that you can have only one network (with normal end-user devices).

Using smaller networks than /64 is certainly possible, but it's not something you want to do on end-user network and it's best left only for things like point-to-point connections and intra-datacenter networks.