If contracts are written correctly (they often aren't) there should be data rights included for most systems. The problem has been that they aren't written correctly, and obtaining data rights after the fact often ends up being much more expensive. The H-60 was a good example of this. Sikorsky wanted substantially more money than DOD considered reasonable so USAF ended up recreating most (all?) of the documentation themselves over the decades.

And with more modern approaches to acquisition, the data rights aren't as readily available because so many systems are COTS. And with someone between the manufacturer and the DOD as the seller, you can't easily insist the manufacturer supplies tech data. They aren't the direct seller.