My theory about the motivation behind the extra security is that it's largely driven by corporations wanting to make desktop Linux less free and less open, and normalize proprietary software instead of open-source. Because profit. Proprietary software is inherently not trustworthy, so the execution environment needs extra security and restrictions, and must be generally less powerful to reduce the damage it can do. Essentially, proprietary software needs the same precautions as malware. So the corps needed to "androidify" desktop Linux. Hence the change from curated distro package repositories to corporate app stores, and the reason why there's so much money pushing to replace X11 with Wayland.
In an open-source ecosystem, users and developers are one and the same, or at least on the same "side", cooperating with each other to make tools which work as well as possible for everyone. Each big program tends to be a collaborative effort where a lot of people contribute to make things better for everyone. Things mostly "just work" and people can typically trust their computers not to do anything weird or hostile.
Very different than a proprietary commercial ecosystem, where users and developers have more of an adversarial relationship. Each program tends to be created in a closed silo by one person or a relatively small team, and is designed primarily to extract money from users, with all other concerns being secondary. It is very common for profit-driven developers to engage in deceptive practices, or do things the user doesn't want, like showing advertisements, collecting and selling data, sabotaging products from competitors, using the device as a node in a botnet or secret compute farm, forcing unwanted updates, microtransactions or subscription fees, etc. So nothing can be trusted, and the entire system needs extensive protections against every type of misbehavior imaginable... even if that means reducing the power and features available to the user.
I've really enjoyed the past few decades of living entirely in the open-source world, where those problems pretty much just don't exist. But with corps pushing the androidification of desktop Linux, I fear those days may be coming to an end.