We find that the avos m'lachos (classes of forbidden activity on shabas) are based upon the labors associated with the mishkan (tabernacle) in the mishna (e.g. Shabas 11:6) and g'mara (e.g. Shabas 97). This question assumes that the construction of the mishkan and not the service therein was the basis for the archetypal m'lachos.
Considering how much metal work the construction of the mishkan and its accouterments entailed, why are there not avos m'lachos like mining, pounding, smelting, and vigorously applying a bellows?
This is considering that we have other m'lachos such as dash and borer that the g'mara seems to tell us are only listed separately because they were procedurally distinct in the mishkan preparation even though they are functionally equivalent. Otherwise, I might conclude that since the acts involved in producing metal structural elements for the mishkan would de facto be forbidden because they fall under other categories they didn't need their own.