How can the concentration of a solid be constant during a reaction?
In physical chemistry one does not use the concentration of a chemical but its activity (see https://en.m.wikipedia.org/wiki/Thermodynamic_activity)

In a solid the standard condition is the pure chemical. In case of the solid block of aluminum it is therefore in its standard condition which leads to the fact that the activity is 1 (the chemical potential is equal to the chemical potential in the standard condition as the block of aluminium is in its standard condition - see the link I provided for the definition of the activity).

Therfore it doesn't need to be considered in calculations like the law of mass action as multiplying or diving by 1 does not affect the result.

However in "real chemistry" one does not calculate with activities too often and uses concentrations instead and in order to apply the same trick one simply says that the concentration is constant.

So in the end it's just a result of sloppiness because one doesn't want to work with activities.