Why do you compare GrapheneOS to a standard Linux desktop, instead of to Google Android?
Obviously, if you take a platform that is geared towards running applications from untrusted sources and compare that to a standard Linux desktop distribution that isn't configure for that, you'll find common desktop Linux lacking.
On the other hand, does GrapheneOS or Android have a desktop that's halfway usable?
Historically, Linux got the reputation for being more secure because Microsoft made a number of brain-dead decisions, some in clear support of a better user experience (arguably, but the omission of 3 letter file type indicators was to not complicate things too much), some just brain-dead (open ports and running services), Linux fanbois boasting and then some real advantages of Linux through the Unix heritage: being a multi user OS with the networking built in and not tacked on afterwards.
All these points are still true for Linux, though the competition has caught up. For the true paranoid, if you need to sandbox, you're doing it wrong.