But the government can unilaterally end other peoples contracts and forcibly acquire property. Why does it then follow that they must follow a contract that they could otherwise end just by giving it away and compulsorily acquiring it back?

Its like a pet bear that is allowed to eat from the floor but not when the meat is dangled over its nose.

Imagine a scenario where the government is building a rail line from A to B, and compulsorily acquires every piece of land on the route, but right smack dab in the middle is land they already own. You expect them to say "Well gosh dern we cant break that contract we signed guaranteeing that this would be a public urinal for the next 1000 years, after forcibly acquiring 10000 other parcels of land at gunpoint, cancel the project". Its entirely unrealistic to expect an entity that is designed to use its land to facilitate society, and can get whatever land it wants to think in terms of some random contract it can break completely legally in any other scenario.