HFT is not "automation" in the traditional sense. Stock markets exist to raise funding for projects (exits using ipo, new projects in public companies). Those occur on the day+ scale, not milliseconds. There's no point to increasing the speed of it (other than just for the sake of increasing the speed). It's like increasing the speed of the channel switching on your tv - after a certain point it's better to devote engineering resources on other aspects of the tv.