I don't agree with the generalization here. Sure, it is generally advisable not to rely on security through obscurity, but depending on the use-cases and purpose it can be effective.

I dislike DRM systems with a passion, but they, especially those for video games like denuvo, can be quite effective, if the purpose is to protect against copying something for a short time until it gets cracked.

Otherwise I agree that software developed in the open is intrinsically more secure, because it can be verified by everyone.

However, many business and governments like to have support contracts so want to be able to sue and blame someone else than themselves if something goes wrong. This is in most cases easier with closed source products with a specific legal entity behind it, not a vague and loose developer community or even just a single developer.