It is the result of the imperfect interaction of the pilots with supposedly failsafe automatics, which pilots are rigorously trained to trust, which in this case failed them
The problem is, the 777 autothrottle was never designed or intended to be a 'failsafe' system. It was not certified as flight critical because the pilot can
always override what it's doing simply by manually moving the thrust levers. Plus, unlike most flight critical systems, there
is an "ON/OFF" switch. If pilots are being taught otherwise, you've just identified a big part of the problem.