Zend certified PHP/Magento developer

Why is it not possible to delete files from a full disk? [closed]

I’ve read that if a disk is full it is not possible to delete files because there is not enough space. So I am genuinely curious why space is needed to perform a simple delete operation when all left to do is to unmark the space allocation of the files so that this unallocated space blocks can be overwritten with new data?
Is freeing disk space a more complex operation than removing some indeces & inodes?

I couldn’t find any posts telling how to solve the problem but not why the problem occurs in the first place. So any linking to the why or answer is appreciated.