I don't know about httpd specifically, but many applications want root only to be able to bind to a privileged port (like :80). This can be circumvented in one of a few ways:

1. add this to .service

  AmbientCapabilities=CAP_NET_BIND_SERVICE

2. or listen on :8080 and use NAT:

  iptables -t nat -I OUTPUT -p tcp -o lo --dport 80 -j REDIRECT --to-ports 8080

3. or make the port unprivileged

  sysctl -w net.ipv4.ip_unprivileged_port_start=80

It may work for httpd too, I haven't tested it.