The statement proves a withdrawal happened. Underwriting is asking a different question: whose money was it before that? An exchange statement doesn't show where the crypto came from, how long you held it, or — if you moved coins in from a self-custody wallet — that the sending wallet was yours. Deposits into your Coinbase account are, to an underwriter, just more unsourced deposits one hop upstream.
There's also a format problem. Exchange transaction histories are often self-generated CSV exports with no account holder name on every page, which fails the basic document-authenticity check underwriters apply to bank statements. None of this means the funds are unusable. It means the statement is one piece of a source-of-funds file, not the whole file.
Notice what actually failed here. The standard advice — get the crypto onto a regulated exchange so the lender can see it — treats custody at a venue as if it were proof. But the statement failed review precisely because where crypto is held says nothing about whose it was or where it came from. Underwriting has never required your assets to live on an exchange; it requires them to be provably yours. The exchange detour bought you a document that doesn't answer the question.