> Because filesystems don't guarantee they will overwrite or wipe any particular disk blocks.

Some filesystems have a richer interface to the underlying storage device, allowing them to invoke commands such as ATA TRIM or SCSI UNMAP - either incrementally as blocks are freed, or on demand - which request that the underlying storage device forget the block contents.

So the necessary interfaces exist and are widely available, and even if imperfect they improve the situation.

> Some filesystems have a richer interface to the underlying storage device, allowing them to invoke commands such as ATA TRIM or SCSI UNMAP

No, that's not a guarantee of data erasure. Not just because it's just a request that the device can disregard, but also because filesystems play tricks (like storing small bits of data inline, or logging data in various places, etc.) and they don't clear all those blocks just because you wanted to clear a couple bytes.