It comes from a history of using mostly trusted application sources like Debian/Ubuntu package archives with manual review being the norm. And few supply chain attacks.What most of these people do not seem to get is that proper sandboxing does not only protect against attacks from the inside (rogue developer, supply chain attack), but also from the outside. Most desktop apps probably have a good number of security vulnerabilities that can be exploited when they parse untrusted data. On the Linux desktop, most apps still use decades-old C libraries for parsing XML, images, JSON, etc.
Sandboxing also protects against external attacks.
Again, Snap/Flatpak provide this model, but we need to see more application authors take them up to ship their software.
Agreed, though for a lot of technical and social reasons, most apps still need privileges that allow trivial sandbox escapes on Flatpak (I don't know or care about Snap). Strengthening app sandboxing should be a top-priority for the Linux desktop, but only a few people seem to care. The same for fully verified boot, etc. Even things like UKIs only go so far, yet almost no distribution has adopted them.
The general security mindset of the Linux desktop community seems to be stuck in the 90ies, levitating between hahah, they cannot get root (as if that matters on desktop Linux) and secure boot and sandboxing is here to take my rights (on open source desktop Linux, seriously?).