The reason why not is very simple: the SHA-1 algorithm was chosen because it made it practically impossible for two commits to have the same hash, not on the assumption that it would remain forever immune to collision attacks.

A vulnerability can be found in a hash algorithm at any time, whereas changing git's hashing algorithm is inevitably a slow process. IMO, if you want to ensure that someone gets an exact set of files, zip it up and sign the archive with the algorithm of your choice. It's not necessarily going to be practical to change git's signing algorithm every time a security issue is found.