Why is it so problematic to have close to zero standby power consumption?
The mobile phone charger is a power conversion circuit which changes your power line voltage (110 or 220V) into something that is useful for your mobile phone (probably 5V). To do this it needs to have some electronic circuity inside which has to be powered and it has to function even if there is not phone around so it can detect one when you connect it.

The charger could be merely a mechanical device like the power socket itself but it would then require all the charging circuity to be inside your phone. Unfortunately it is quite big and relatively heavy so it would be inconvenient to carry it around all the time.

Regarding the actual 30mW figure: if instead of mW you consider the currents involved you arrive at around 300μA (30mW at 100V). This also means a resistance of \$330\,\mathrm{k\Omega}\$. It is quite difficult to work using resistances higher than and currents lower than this while still having to sense the moment when somebody plugs the actual load.

OTOH 30mW is really, really small. The vampire current draw problems are not as important as many believe. If you want a good review of many aspects of this then I suggest reading "Sustainable Energy – without the hot air", especially the chapter on this topic