They used to be small. They've grown approximately at the same speed as HDD sizes, which is significantly faster than throughput, not to mention seek time.
A consumer HDD from c. 2000 might have a 2 MB cache and 40 MB/s throughput, so a full cache empties from sequential data in 50 ms best case. Current 2-3 TB drives have a 64 MB cache and 100-150 MB/s throughput, so a full cache takes around 500 ms minimum to empty.
For non-sequential data it's much worse.
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