When all features are enabled, why does DNSSEC does not have an option to be enabled?
To be able to use DNSSEC, your server must be using PowerDNS. You can check to see if your server is using PowerDNS through WHM in Home >> Service Configuration >> Nameserver Selection
All servers in the DNS cluster must run PowerDNS if domains that use DNSSEC exist in that cluster. cPanel & WHM supports DNS Security Extensions (DNSSEC) in DNS clusters. PowerDNS servers with domains that have DNSSEC configured can configure DNS clusters.