Because they did not.
WWI ended in an armistice. There was no surrender.
In fact, it was an armistice of over 7 months, lasting from November 1918 to June 1919, when they finally got together to formalize the end of the war. However, during that time Germany was wracked with a revolution, and the government in charge at that time was only barely in charge. The German soldiers had already been pulled from combat, and most had been released from the military. Germany never did surrender, but the Allied Powers did force through some major concessions, as they did still have forces on the border.
In short, Germany pretty much had to agree to giving up territory or risk being invaded. But they did not "surrender".
There is a reason why I bring this up whenever some moron tries to talk about the Japanese attempts to end WWII in 1945. Because Japan was trying the same thing, not a surrender but an armistice. But the Allies learned their lesson after WWI, which is why they refused to consider an armistice and would only deal with a surrender.
But I welcome Tiger to show us the "Instrument of Surrender" of Germany from WWI. There is none, only the "Treaty of Versailles". Which opens as follows:
"Treaty of Peace", to make the armistice of 11 November 1918 permanent.
There was no surrender, just a state of permanence on the previous armistice.
In fact, the word "surrender" only occurs way down in Article 169 and lower, where it talks about the Central Powers surrendering excess military supplies.
And I have yet to see Tiger provide any proof of any of his claims. None, it is actually rather funny to be honest. Especially as I can see no way it is possible to realistically confuse a surrender with an armistice.
The Korean War ended with an Armistice. Neither side surrendered.
The Vietnam War ended with North Vietnam suing for peace, and in the Paris Peace Accords agreeing to never invade South Vietnam again.
Which they then did 2 years later.