The localhost alias is defined in the first line of /etc/hosts. Perhaps some people have incorrect hosts files.
head -1 /etc/hosts
127.0.0.1 hostname localhost localhost.localdomain localhost4 localhost4.localdomain4
By the way, to access cryosparc locally or over a SSH tunnel on CentOS, no additional configuration is required. To access remotely, you need only open that one port (38000 by default). I always leave SELinux set to enforcing.
Any actual SELinux policy violations will be reported in the system journal, along with one-time instructions to permit that action. Typically these are useful to guide you to the correct generally policy change, but again, there’s no SELinux configuration required for cryosparc.
I also have cryosparc run under its own user, and all cryosparc associated directories have SELinux type user_home_t.