Answer:
Because contrary to a century of myth and propaganda, Versailles didn't go far enough; in 1919, Germany was more powerful relative to its neighbors than 1914.
Economically; in 1918 Germany was starving but its industrial base remained intact. Unlike France, whose main industrial zone was the front line for four years, or Belgium, whose entire infrastructure was blown up by the retreating German army in 1918 (yet another reason for the lack of sympathy at Versailles).
The hyperinflation of 1922–1923 was largely manufactured by the Weimar government; it wiped out personal pensions, savings and investments but benefitted large companies by doing the same to their debts. This is one reason over 80% of the top 50 German companies in 2018 were founded in the 1840s or 1850s.
Politically; Thanks to Woodrow Wilson’s principle of ethnicity = nationality, Versailles surrounded a largely intact Germany with a host of weak satellite nations, cobbled together from groups that didn’t like each other very much (eg Slovaks and Czechs), with large resentful minorities (Sudetenland Germans, Ukrainians in Poland, Hungarians in Romania etc.) and indefensible borders.
Militarily; they found ways around the limitations on the Reichswehr. These included funding veterans associations, eg the SA, Stahlhelm, Reich banner Schwarz-rot-gold, but which also provided political parties with their own militia and thereby undermined the Republic.
They also signed secret agreements with Russia, for training and exercising aircraft, tanks and tactics but most importantly retained defensible boundaries (unlike their neighbors').
This meant the Allies weren’t going to make the same mistake in 1945. Weakening Germany was not simply a function of Stalin; even in 1990 (pre-German re-unification), then French President Francois Mitterrand said ‘I love Germany so much, I’m glad there are two of them.’ Thatcher made the same point but not nearly so elegantly.
Explanation:
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