Silly question about secure deletion of files

[Faramir]

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Well, I already know why it is not reliable to "securely delete" a file,
I understand the hdd can do the "overwrite" process in the "wrong" place
of the hdd.

But yesterday I had to defrag my hdd, and that made me wonder: why we
can't overwrite a file, but we still can defrag the hdd? Is there a
hardware feature that allows SO to defrag the hdd?

 Best Regards
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