And to be clear, there's a good reason for it: it's to prevent corruption.
If things aren't overspecified and providers aren't treated like a commodity, then it's incredibly hard to prove that a government official actually awarded a contract in a fair process, rather than just sending it over to their best friend's business.
Unfortunately, nobody's really come up with any reliable process for having the flexibility to get good products for good value, while reliably preventing corruption. And when there aren't these ironclad protections against corruption, experience shows it turns endemic, so much money flows through the government.
It's a seriously tough problem.
The reason it doesn't exist in the private sector is that the chain of accountability from managers to CEO to board seats is actually quite strong, and shareholders are incredibly motivated to extract profits. The accountability to voters in a democracy, on the otherhand, is far, far, far weaker -- as voters vote primarily along party lines or on only the absolute biggest hot-button issues.