The default setup of Docker does not make any assumptions about the host setup, so it assumes it might only have one IP address, so there is only one set of ports.
It is perfectly ok if you have lots of IPs to put routed IPs on the `docker0` bridge, and never use port publishing at all, or to use some of the other optional setups, such as the new macvlan and ipvlan setups https://github.com/docker/docker/blob/master/experimental/vl... which are the kind of production setups you may want if you run your own networking. But Docker cannot assume anything about the network setup in the default configuration, hence the use of published ports, which is kind of inconvenient but always works in any environment.
It is perfectly ok if you have lots of IPs to put routed IPs on the `docker0` bridge, and never use port publishing at all, or to use some of the other optional setups, such as the new macvlan and ipvlan setups https://github.com/docker/docker/blob/master/experimental/vl... which are the kind of production setups you may want if you run your own networking. But Docker cannot assume anything about the network setup in the default configuration, hence the use of published ports, which is kind of inconvenient but always works in any environment.