Loader can enter the tank tired, hungover, with stomach cramps... They also don't work that well when tank is going at high speed over uneven terrain. Human loader is slightly faster at optimum circumstances, but optimal circumstances rarely happen in combat situations. The main reason why Abrams doesn't use autolader is that these systems were still unreliable when they designed the tank and it would take complete redesign and rebuild of the tank to fit the autoloader now, so there is no willingness to do it, for little benefit it offers.
The biggest benefit of human loader is that you have a fourth guy to help with routine hard work around the tank. Keep in mind that later Western tank designs like Leclerc, K2 and Type 10 all use autoloader and so will the Abrams successor 20-30 years from now.