This reason is that many users don't have a second Pi-hole. If we'd show them a box to enter another DNS server, most wouldn't know how to handle it. It is likely that such a feature would cause more harm than anything else. I'm not talking about you, however, the typical user might be touching networking the first time at all when they install Pi-hole so this a valid issue.
This is very possible with Pi-hole, however, for the reasons I mentioned above, is not directly available on the web interface.
Create a file /etd/dnsmasq.d/99-second-DNS.conf and enter
dhcp-option=option:dns-server,192.168.0.2,192.168.0.3
(replace the IPs by the local Pi-hole's and the alternative one's). Restart pihole-FTL with sudo pihole restartdns to have it re-read the config files.
Devices that offer this are typically not DNS servers themselves. Routers, for instance, either allow you to set two DNS servers they distribute (the clients will use those DNS servers directly, not ask the router) or use this addresses as their own upstream configuration (in this case clients will only receive the router's IP address as a single DNS server address).
Our DHCP server has never been meant to be a solution that can support more than very basic network topologies. It is meant to be useful for those who cannot change the DNS servers on their ISP-provided routers to have an easy alternative. We still offer a lot of functionality and power, however, most of it is not exposed to keep the interface simple.
We are still here to assist anyone to get the configuration they'd like to set up. And -- often enough -- it turns our that users can achieve what they want with something much simpler than they initially envisaged (This is not the case for you, however, it is still what we see most of the time).