Given a file name, it is nearly trivial to find the i-node and other information (that is part of the purpose of a file name, after all). Given an i-node number (and device number), finding the file name(s) requires searching the entire file system (once you've determined which file system from the device number). That is, the mapping is one-way.

And an i-node can have no file name or can have hundreds of file names.

Why? Because the mapping needs to be efficient in the one direction but the "need" to go the other direction doesn't justify the extra overhead and complexity so most file systems don't implement the other direction.