Because the ZIL is a journal log, not a cache. It is intended to increase data security without sacrificing too much performance. Many people also confuse ZIL and SLOG devices though...
By default, the ZIL is written on the same disks as where the data will be stored, but an external device (aka SLOG) can be added. From that point on, your write IOPS will be limited by this SLOG device, so normally you add a more expensive fast disk as SLOG device to increase your write IOPS.
By default, the ZIL is written on the same disks as where the data will be stored, but an external device (aka SLOG) can be added. From that point on, your write IOPS will be limited by this SLOG device, so normally you add a more expensive fast disk as SLOG device to increase your write IOPS.