A bank-wide view of counterparty credit risk (CCR) for economic capital calculation is challenging primarily because it requires the aggregation of immense amounts of transaction data across numerous portfolios and counterparties in real time or near-real time. This involves large-scale gathering of data, netting agreements, collateral management, and continuous transaction monitoring, which can easily strain both IT infrastructure and human resources.

Choice A is correct because consolidating and monitoring exposures across all divisions and products demand enormous data processing and monitoring efforts, often testing the limits of a firm's human resources and operational capabilities.

Choice B is incorrect. While expensive software may be utilized, the fundamental barrier is the vast scale of data gathering and processing, not merely the cost of software.

Choice C is incorrect. The reliance on credit rating agencies deals more with estimating probability of default rather than the bank-wide aggregation of counterparty exposures itself.

Choice D is incorrect. Although autonomous divisions and organizational silos present practical difficulties, the primary and most resource-intensive challenge lies in the large-scale data processing and monitoring requirements necessary for a unified view.