Why can't I get SRV records populated when querying domains with DNS Lookup API?

SRV records are used to identify computers (i.e., subdomains) that host given services, whereas DNS Lookup API provides only information about the root domain name (the API doesn't cover any record types for subdomains).

We have a separate service that can give information about all subdomains known to us, but only their names - https://domains-subdomains-discovery.whoisxmlapi.com/. 

This way, you can get all available subdomain names and DNS records data for each separately.