They say it, but they're wrong. Historically speaking there have been basically about 2 fascist governments, and they fell because they lost wars. And Germany, for one, did run them with high competence, to the extend that it took years for many countries to do anything about.

It we loosen "fascist" to just mean any authoritarian government, there are many that run of very long time.

WWII started in 1939 and was done in early 1945, so it didn't take that long.

More importantly, maybe the Nazi's were competent at first, but they absolutely fell apart internally due to mistrust, back stabbing, and demanding of loyalty above all else. Hitler famously made many poor military decisions.