Why weren't Gigai used for Spearhead?
Two words: Plot armor.

All jokes aside, this is never really explained.  It is touched upon, however, in the anime during the non-canon Bount filler arc.  The three special mod-souls created by Urahara to help Ichigo regain the power of his bankai, all used their own custom made Gigai.  Once again, this arc is non-canon and so are the characters.

This one may never have a satisfactory answer.