The military doesn't have the luxury of things being unreliable. It puts a pressure on them that corporations don't necessarily have: they'd rather have a less-effective but proven system than a potentially-more-effective but riskier system (especially since each system they have comes with massive logistics support).
Ironically, corporations can afford to take more risks of failure (financially and project-wise) than militaries because failure for them doesn't mean actual human death (and when it can, you see processes come in that look a lot more like military processes).
It's actually the commercial/consumer side that gets more reliability than the military side.
The military should have very reliable systems, and they often know the point at which their systems will fail (MTBF calculations are easier to develop with their record keeping). However, the military also has an almost unlimited budget and body count to keep just reliable enough things working much better than they should. It's also really bad about actually competing companies against each other.
The commercial sector, targeting consumers, is where you actually get reliable systems. Why? Because consumers will go towards either the cheapest option (reliability is replaced with ubiquity in the market, it's replaceable) or the more reliable but more expensive options. They (individuals) don't have an unlimited budget or unlimited time to maintain everything in their life. There's competition in the commercial world that's completely absent in the military world
The two major exceptions are where COTS products have taken over (definitionally, DOD is using commercial, often consumer-targeted, products instead of military specific products) and special forces. Special forces often bypasses normal acquisitions processes and so ends up having a better chance to compete vendors against each other than other parts of the military.
This doesn't mean everything the DOD procures through normal acquisitions is inherently unreliable, but reliability is only one of many factors and often only really discovered after selection and full-rate production has started. By that point, the DOD is committed to it for years to come. Each DOD procurement is separate enough from others that you don't even get huge opportunities for reuse. The F-35, to pick something from this century, didn't get components that were shared with other aircraft in the DOD fleet. It's almost all new, which means a lot of things were learned about its reliability after it started flying. It has new comms, new radar, new almost everything. Even the engine (though that probably used many subcomponents shared with other engines) was a new engine just used by the F-35.
Ironically, corporations can afford to take more risks of failure (financially and project-wise) than militaries because failure for them doesn't mean actual human death (and when it can, you see processes come in that look a lot more like military processes).