UNIX philosophy.

Why is the opposite of cat tee, and it's not even a proper opposite but also does a different thing (outputs to stdout) (which is ostensibly its real purpose)?

Also, why do I even need the opposite of cat (and cat itself while we are at it) when those needs should have been served by input and output redirection, but turns out that the latter is completely made up and broken shell bullshit: you can't cat <file1 <file2, and you can't EDITOR='echo zzz >>$1' visudo for example.

Orthogonal design, my ass. Do one thing and do it well, my ass.

The opposite of cat, tac, applies strong suction to its output stream causing data to backflow into the input stream.

E.g., < input.txt tac | rot13 > output.txt writes the rot13 of output.txt to input.txt.