Because speed is an implicit and intrinsic "figure of merit" in automated market making: if you are faster than other market makers, you outcompete them.

There are two straightforward problems with microsecond-speed electronic trading:

* At very small timescales, possibly as a sort of inevitable consequence of the CAP theorem, correlations between instruments that should trade in lock step start to break down. Since you can make money correcting these mispricings, time and energy gets sunk into doing that, and that imposes costs on the rest of the market. It's uncertain how high those costs are, but they are small, and clearly a pittance compared to non-automated market making.

* Electronic trading provides very nice paychecks for pretty interesting technical work, which means some talent gets attracted to the market that would otherwise get paid less money doing something with more social value. How big a deal this is to you depends a lot on your worldview.

To me, neither of these issues seems worth the galactically high cost of restructuring all the markets in such a way that we'd reliably avoid these problems.