Why is it that Anti-Virus software cannot repair files that are infected by a Trojan or a worm?

Strictly speaking, there is no such thing as “file infected by a Trojan or a worm”. One difference between a virus and a Trojan or a worm is that a virus will replicate itself to a clean file, which will in turn infect other clean files when the infected file is executed or opened.
A Trojan is a malicious program installed on an infected computer which does not attach itself to any file. Worms are also malicious software that spread across networks but do not replicate onto a clean file. Therefore, there is no file to repair when it comes to a Trojan or a worm.