A mining ships doesn't dismantle themselves to build a mining station, so why should constructors have to?
Mining ships have asteroids from which to make the bases. Constructors don't have anything to use apart from their own hulls.
That's it exactly. Think of it this way, divide the mining module into three pices-25%
miner/25% constructor/50% ore processor. That's why miners don't destroy themselves
when they build or upgrade a base on an asteroid, they use what they take from the asteroid. Constructors on the other hand don't have anything but themselves to use for building bases (ie. resource, econ, mil, culture). For resouce bases(econ,mil,cult, money,etc.) the constructor is building an enegry collector more or less. For the actual base (mil,econ,or cult) it is building a means to disperse the collected energy. Due to the way constructors work they don't have the space for extra parts. Now I know what you'er thinking " What about the miniaturizing techs?", right? Unless the great and mighty dev overlords,(all hail the dev overlods), deside to add a "parts/supply" module to go with the constructor, which I doubt, there is no room for "extra parts". So constuctor ships dismantle themselves for the parts to make the bases. If that's not a good enough explanation try this, it's how the devs want it to work.