The reason is security. It isn't much of a problem on a laptop only you use (since then your home directory is likely none safer), but on a shared machine, or a

The reason is security. It isn't much of a problem on a laptop only you use (since then your home directory is likely none safer), but on a shared machine, or at least a shared file system, other accounts can access stuff in /tmp just fine, e.g. by replacing the file with a symlink. To perform a delete operation on a file, you only need execute permissions on the containing directory (iirc), but nothing on the file itself.