Why should World War II have started with rapid major victories by
Germany and Japan and have ended with the crushing defeat of both
countries?

Conquering huge territories is one thing. Holding them over a long time another.

Scriptor

You've probably never played the classic wargame Axis and Allies,
which provides one answer. In this game, the Axis starts off with a
larger military force, which provides them with a greater advantage in
the short term. The Allies, however, start off with with a bigger
economic capacity, which gives them more of a long term advantage.
Therefore, if the Axis player wants to win, he has to do it quickly,
before the Allied player's economy can ramp up to pump out a lot of
units.

Basically, you're asking what happened with the whole war. Both
countries attacked nations that were inferior to them at the time of
their attack, but over time the Soviet, British, and US war machine
succeeded due to their vast resources, technological innovations like
radar, better commanders (much of this is due to Hitler's
interference), and sheer manpower. Japan clearly suffered from a lack
of resources against the US and made a key strategic error at Pearl
Harbor by not damaging enough US aircraft carriers; the war was over
for them after Midway. Italy wasn't a serious power. Germany failed
because it didn't knock out the UK, had a ton of territory to manage,
and although they were probably strong enough to defeat the USSR,
their attack began too late in the year. Stalin succeeded here due to
his massive army, newly developed tanks that exceeded any German
armor, better use of his commanders, the Germans weren't ready for the
cold winter, and the Germans mistreatment of the locals hurt them. I
think most people today forget how bad the situation looked in the
USSR; even one year after the initial attack many in the West felt it
was only a matter of time before the Soviet Union fell.