Probably because of the "D" in ACID transactions, so the transaction log cannot be meaningfully write-cached. OTOH, writing to tables/indexes is already done "

Probably because of the "D" in ACID transactions, so the transaction log cannot be meaningfully write-cached. OTOH, writing to tables/indexes is already done "out of order" and aggressively cached in the buffer pool, and flushed to permanent storage only occasionally (and relatively rarely, e.g. SQL Server does it approximately once a minute).