The buffers maintained by the OS and drive are totally different, the entire point of having huge buffers in the device is so that it can agglomerate and reorder otherwise totally unrelated writes into flash banks it is presently convenient to write to.

When the OS flushes stuff to the device, it is done such that metadata writes reach the disk in an order that ensures a recoverable filesystem image on power failure. If the device does not preserve that ordering all bets are off. The mapping that goes on inside a SSD is of sufficient complexity that it's much easier to simply guarantee the device has enough independent power to ensure all acknowledged writes are persisted after acknowledgement, and that means capacitors, or best case, data (e.g. committed, acked DB transactions) that magically disappear during a power loss event.

You cannot have a device that temporarily writes to RAM and tells the host computer "OK that's on disk now" (i.e. every SSD in existence with a throughput exceeding a few MB/sec) be reliable without an independent source of power to ensure that RAM reaches flash in every situation