[quote=Legend]
That's so much land, mass, space, gravity, time. Why bother mining planets? You'd literally have to find a small galaxy of solid iron, at least.
[/quote]

Land- Years of mining operations will be sufficient, and it'd be harder to find a small galaxy of solid iron than to just mine up everything you need elsewhere.
Mass- Same.
Space- Void.
Gravity- Gravity control mechanisms are in place to keep everything from falling apart... Or together.