Failsafes are triggered irregularly and are thus prone to bugs. It is also very rare to go back and review data with a separate implementation of the failsafe to see if the normal implementation is functioning incorrectly in certain circumstances, so errors in the failsafe algorithm can be difficult to detect until a major problem occurs.

There's nothing preventing humans from abandoning the markets as well. This isn't unique to HFT. Human market makers can also withdraw their liquidity when things don't look good.