Wow this isn't rocket science
Archer/Shirou cannot trace Excalibur(WoG): they can make something that looks like it and farts rainbows, but it's straight up not excalibur. It's like if you look at a statue you thought was made of, I dunno, copper online and then you made a statue that sorta looked like copper, but the original was made of bronze. You didn't copy it successfully, but you created something that looks like it, yet that's all it really does: look like it.
Note that I have no idea if you can make statues out of copper or whatever shit