I think the main reason that GNU tools are overly optimized is copyright. [0] They just try to avoid any copyright claim by the old proprietary UNIX tools like the copyright on an empty file. [1]

[0]: https://www.gnu.org/prep/standards/html_node/Reading-Non_002...

[1]: http://trillian.mit.edu/~jc/humor/ATT_Copyright_true.html