For one, you wouldn't have the ability to use a crane while demolishing/excavating/building under an existing structure. That would prove to be pretty complex and it would take much longer than constructing a traditional building, both of which would add significant cost to the project. Further, integrating a new building into the West building would require rebuilding many of the common facilities (i.e. elevators) so that they could share a lobby -- not sharing a lobby, there would be no point to doing this.
That said, the space could be put to better use than a parking garage but as it stands now, several improvements could be made to straighten out the confusing layout and give the building a pleasant look both inside and out.