Why is it difficult to compare IAS 18, Revenue, to U.S. GAAP?

The IASB definition of revenue is very complicated, whereas the definition of revenue under U.S. GAAP is straightforward.

Revenue is not defined under U.S. GAAP.

IAS 18 defines revenue in a variety of ways, and not just cash.

U.S. GAAP has several standards that deal with revenue.