Because it only set things up for round two. That's why. There are treaties that you simply don't do and Versailles is a purely revenge-driven treaty, the sort that only causes problems down the line. Especially with a country whose history is filled with attempts of unification that got destroyed by the French (and at times British).
Remember that Winston "I'm a near Tillman Racist" Churchill outright said that the Treaty of Versailles was a fucking bad idea.
Not really. The Treaty of Versailles was to strip Germany of as much power as possible (and thus put it into a position that any invasion would mean it'll be run over and unable to pay its debts), or have you forgotten how the French and British stripped the Rhur and Rhine of industrial capacity in the years beforehand? The very industry that Germany was planning to repay the respirations with via their products?
You two have forgotten that Germany united because of the French (and in part, the British) fucking them over repetitively historically. Every time they tried to unite, the French (and at times the British) toppled every attempt until Franco-Prussia.
Yes, it was a 'revenge-treaty.' Because Germany not only lost the war, it also spent almost the entirety of the war wrecking the French countryside, because that's where it was fought, and it also committed a large number of war crimes. Nothing on the order of WWII, but some of the shit they got up to with the Belgians was nasty.
Paying the other side when you lose has been a part of war throughout all of history, until the US became the world hegemon and decided that was unethical, and trying to rebuild the defeated nation(s) was a better plan.
Disarming Germany was also to be expected, given what it had done with its industrial capacity during the war. You don't leave an enemy ready to just start the war up again as soon as the US has pulled its troops off of the continent and had another election cycle turn it more isolationist again.
Further, while the web of treaties and the like that sent the conflict spiraling out of control aren't something you can reasonably blame Germany for, you can absolutely blame them for continuing the war year after year. They had explicitly, clear ambitions to carve up their enemies and keep them from becoming peer threats again if they won, and as they held the upper hand early in the war, then a somewhat balanced position for most of the war until the US joined, they could reasonably have proposed a peace that simply involved returning to the old borders, let's please just stop all the killing.
But they didn't. The German leadership was set on becoming the Hegemon of Europe by force of arms, and they didn't negotiate a surrender until it was absolutely clear that they were going to lose militarily due to America decisively tipping the scales, in spite of the Russians pulling out of the war. If they had been willing to sue for peace when the outcome of the war was still in doubt, they could have gotten better terms.
Instead, they continued to fight, inside of French territory, trying to hold onto the dream of ascendant Imperial Germany, until their position deterioriated to the point where Versailles was the best deal they could get.
This is what happens when you spend years despoiling the territory of your neighbors, and trying to break them. If you lose, you get broken instead.
Note, I'm not trying to argue there weren't unsavory or immoral things the allies did as well. There were. But after the first year or so of the war made it clear that people were just being sent to die to try and claim another hundred yards of cratered hellscape, they were in position to end the war at any time, and they refused to do so.
Imperial Germany played a stupid game, and they got a stupid prize.