They can be used by anyone based on R's package-manager GUI.
Research in most fields is going to move more quickly than any software team can add functionality to monolithic programs like SAS. Therefore, most functionality becomes available only in R. Users - people who want to run specialized-domain-function-X on their dataset are best suited by learning R, then using their colleagues packages.
I don't know about the package system for SAS or SPSS (though I am aware that papers get published with code for said platforms), but I think the bigger barrier is availability; R is free and open.
They can be built by a single prof/grad student.
They can be used by anyone based on R's package-manager GUI.
Research in most fields is going to move more quickly than any software team can add functionality to monolithic programs like SAS. Therefore, most functionality becomes available only in R. Users - people who want to run specialized-domain-function-X on their dataset are best suited by learning R, then using their colleagues packages.