Because we do not have the foggiest idea how to make 'imaginary' mass! And it seems, neither does Mother Nature, because we have never detected any evidence that tachyons are 'real' particles. Just because a particular theory says they exist, doesn't mean they do. It wasn't mathematics that showed us how to travel faster than sound. It was daredevil test pilots and a lot of incremental aeronautical engineering that let us 'sneak up' on this technique. There is no evidence that nature has provided us with a single phenomenon whose 'trick' we can copy. Quantum processes are no help!