If you want to see the source code of Photoshop, you actually need to work for Adobe. Otherwise, you need to be some kind of freaking retro-engineering expert.

What you're describing is known as "security through obscurity", the practice of attempting to increase security of a system by hiding the way the system works. This practice is highly discouraged, as it is known to not actually be effective at increasing the security of a system.

Security by obscurity alone is discouraged and not recommended by standards bodies. The National Institute of Standards and Technology (NIST) in the United States recommends against this practice: "System security should not depend on the secrecy of the implementation or its components."

https://en.wikipedia.org/wiki/Security_through_obscurity#Criticism

Isn't that actually also helping hackers?

No, by sharing the implementation details of the system, it helps those trying to keep it secure by allowing anyone to inspect, discover, and contribute fixes to security flaws.

Open-source software is not perfect and is suceptible to security flaws and vulnerabilities, but it is better and more secure than closed-source software in every way. Every risk that applies to open-source software also applies to closed-source software, but worse.