Git signatures are designed for cryptographic security, and they (currently) rely on the SHA-1 hashes.

When you're signing a commit (or a tag) you're just signing the commit (or tag) message which includes the SHA-1 hash of the relevant "root" tree object (or commit object). The tree object, in turn is (in effect) a list of SHA-1 hashes of the files directly in the directory, along with their file sizes and permissions, plus the hashes of the tree objects corresponding to any subdirectories.

Consequently, if you replaced a file with another having the same SHA-1 hash (and the same file size — a considerable complication), all the hashes would remain the same and the signature would still be valid.

Obviously, once git transitions to SHA-256, the problem will disappear.