IT outsourced services represent an intent on the classic "make vs buy" decision. Organizations choose to "buy" a service than doing it in-house. Of course there are variations where organizations may also choose to keep some services in-house (which happens most often) but largely that's the intent.
In doing so, organizations have few control levers than the contract as they embark on such a partnership. The contract seems to be the only sane way to keep control on the provider for the next five or seven years (or whatever is the duration of the contract). However organizations often miss out few key things:
1. It is not possible or feasible to document each and every aspect of expectation and scope in a contract as several areas may emerge as services mature or as the service provider's (or providers') operations are understood
2. Needs of business will change and evolve over the period of the contract due to business climate changes, market changes and several factors which cannot just be foreseen so far ahead as the duration of the contract. These just don't include the scope but also the SLAs, engagement model, delivery model etc.
The more successful relations have seen a start based on the framework provided by the contract but then the relationship should mature based on mutual trust and understanding. In the later years the contract may just be an over arching framework to be referred in case of deep disagreements but largely the teams working together need to understand each other and make things work than getting the lawyers to do the same.