Binding to Wikibase Datamodel services introduces subtle and spread out, but large costs. The reason is that this added another consumer to that library which increases the cost of making (breaking) changes to it.
The suggested approach is to review which code is actually needed from these libraries and then copy that to code to WikibaseSchema. The rationale is that even a few hundred lines of extra code are less extra cost than binding to Wikibase Datamodel and Wikibase Datamodel Services.