For the F-35, this is roughly the equivalent of asking why Google or Facebook doesn't use COTS software instead of mess of proprietary legacy code. The F-35's

For the F-35, this is roughly the equivalent of asking why Google or Facebook doesn't use COTS software instead of mess of proprietary legacy code. The F-35's reason for existence is its software. Other existing aircraft are faster, can carry more, fly further, or be more stealthy. The F-35's value statement is that its pilots will know more about what's going on around it than any other combat aircraft, for the next ten or fifteen years. COTS stuff is used at the base layers of military avionics. LynxOS is used under the hood of the Army's very successful CAAS helicopter avionics system.