Every mil company designs unique bespoke things often with some exceptions, and even the "open systems" mostly aren't open by any stretch of the definition.

Obscurity is part of the allure, not a repulsive factor.

You don't want a hostile nation suddenly and easily understanding your systems.

On the other hand, its good to design systems where it wouldn't even matter if the hostile nation state knows exactly how it works.

This is a principle in cryptography, but carries nicely to other things.

Asterix is an actual standard for radar data, coming from civilian radar systems. I wish it was updated somehow for 3d beam-steered (and moving) sensors, but still it covers most of the industry's needs... I keep hearing so many grumblings of replacing it (it is binary, unforgiving, quirky, very 80s...) and of course fragmentation is back in.

I have to assume that so many people require access to the specs that an interested party can surely acquire them.

World of Tanks has repeatedly had military personnel updating the technical specifications with details they have found in the secret manuals.

Hat sounds like a particularly weak form of security by obscurity

Also you do not want them to benefit from your smart design choices. You want them to suffer from their bad choices and not learn from your success. (Yes, of course you could have bad design choices that you don’t realize someone else has solved more intelligently.)