Because there is no computer system in the world which is 100% safe from attack or bugs or vulnerability, and every additional piece of software present is a potential attack surface.
OK, fair point.
If someone gains physical access to my airgapped Linux computer which is encrypted at rest, then I'm fairly confident they would find it impossible to extract any meaningful data from it. If someone gained access to an airgapped computer with Windows 10 on it, then I have no idea what kind of bugs and vulnerabilities they could exploit.
Why did you compare an encrypted Linux OS with a Windows 10 machine without encryption? Or maybe you meant to say a Windows 10 PC with an encrypted disk. Do you have reasons to believe that a full disk encryption by the same encryption software used on a Win 10 is easier to circumvent (due to all previously mentioned software and bloatware) compared to one done on a Linux OS? If not, your first line of defense, (the encryption software) seems equally strong/weak on both OSs.
It depends who that someone is. A regular thief looking for something valuable to steal and sell. You should be protected against those.
A well-prepared and determined robber who knows why he is targeting you and what you may have on that computer. Now it's a bit tricky. You could reject his demands, but there are also two knee caps and your family that you need to consider if he knows about them.
If you are targeted by a government agency and tell them no, they will find ways to violate your rights in ways you didn't think were possible in the name of national security.
Regarding the part where you mentioned vulnerabilities in software on Windows. Are there no similar cases to the ones you mentioned on Linux? I don't use Linux, so I really don't know. But the fact that something is open-source doesn't make it safer and more secure. I don't mean Linux, I am talking about any software. Open-source means that you can inspect the code. A codebase that can still suck, be bad, backdoored, and full with bugs that can be easily misused by those who know how to. A badly coded open-source software presents a greater risk than a badly coded close-source software if you think about it. You have open code that is public, compared to hidden code that you don't see.
Every piece of unnecessary software and bloatware is a new risk. If I'm going to the effort of physically removing hardware to create an airgapped device and installing a clean OS, then why would I not pick the OS which is more secure and has fewer vulnerabilities? It's a no brainer.
If you know how to work on Linux or you have the time and patience to learn it, sure, I see no reason to use Windows. The only plus side is its easy of use and greater offer of software.