Why is the file data.txt empty after executing sort data.txt > data.txt?

The file was not empty before the command; the truncation caused by the redirection operator is what emptied it.

sort is designed to sort text files; it works correctly on plain text and the file type is not the issue here.

sort does not detect or handle the case where input and output are the same file; the shell's redirection mechanism, not sort itself, is responsible for the truncation.

When the shell processes sort data.txt > data.txt, it sets up the redirection first by opening data.txt for writing and truncating it to zero bytes before sort even starts. By the time sort opens data.txt for reading, the file is already empty, so there is nothing to sort and the output written back is also empty.