Nobody said there has to be one layer of sandboxing. Defense-in-depth approach is best. The developer can utilise sandboxing to limit the application's runtime

Nobody said there has to be one layer of sandboxing. Defense-in-depth approach is best. The developer can utilise sandboxing to limit the application's runtime abilities to what it should have, and the end user can further restrict the application to their preference using chroots/containers/jails.