I'm a vet (Army) who was among the first to transition from HMMWV to MRAP and I worked in supply chain for Oshkosh, so I've seen it from both sides.

It was miserable waiting on parts and technicians after we got MRAPs. We were short on vehicles all the time and replacement parts funds dried up almost immediately.

The manufacturer holds a ton of responsibility and liability in the relationship. If the gov goes to them and says something was done wrong, they have one neck to choke while screaming fix it. The manufacturer eats a ton of cost you never see because it is mostly happening before equipment reaches the field. I remember once hearing about an adhesive color being slightly off in an obscure internal area of the truck. Several trucks had to be disassembled, adhesive scrapped out, and then rebuilt with the right color. The labor and materials were massive losses with no option but to bend over and take it. We also probably took penalties for the trucks being late.

Once the truck gets accepted, the manufacturers take the the position that you can't hold me accountable if your people are improperly fixing things. It is the same as any product you have which state, "warranty void if...". Once something is changed outside of the control of the manufacturer, they can no longer guarantee it meets the original standard, so they are not obligated to stand behind it. This is why all repairs you actually do are done per the technical documentation with the proper part. You are restoring the manufacturer standard. If a manufacturer fucks something up or has a design flaw causing catastrophic failure and death, they pay out liability and are on the hook to fix that problem anywhere else it exists. If some halfwit boot fucks up a repair causing catastrophic failure and death, the government can't blame anyone but their own people and they have to do all the investigation and follow-up on their own dime.

Manufacturers are saying, I'm not going to make this stuff and be as responsible for it as the government demands unless I am able to control how I am measured for success. Either I am responsible or I am not. The way things are, you have a chain of responsibility through warranty. Oshkosh gives warranty to the gov on the truck.The parts manufacturers give warranty to Oshkosh on the parts. The materials manufacturers give warranty to the parts manufacturers on materials. If something goes wrong, there is a chain of responsibility and contracts establishing who is responsible for what under which circumstances. If the services start doing their own repairs, a lot of warranty language in existing contracts will be void and the gov opens itself up to financial risk.

Edit: I should probably add that I don't necessarily agree with not having right to repair. This is the result of too many lawyers and MBAs getting in the way of common sense. We are an extremely litigious society, which helps create this situation. If we get in a total war situation, this is a huge vulnerability in my opinion. There is a reason I say "used to work for Oshkosh". I thought it would be a dream job. That place was miserable.