Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
The answer is simply because that's the way HGTV (and most other cable-type channels) set it up. If you aren't subscribing to a provider that pays part of their subscription fees to HGTV, then HGTV isn't getting paid. They aren't in business to provide you their programming for free.
If you don't want to subscribe to a cable/satellite provider with HGTV there are some streaming providers you can use. Ones I know of are Philo, Sling, FuboTV, YouTube TV, and Hulu Live TV. There may be others. None of them are free, but they generally cost less than a full blown cable or satellite subscription.
Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi LivRm - Samsung UN55ES6100
Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi BedRm - JVC EM37T
Roku 3, 4200X, Ser 4124C7087583, wifi GstRm - Sanyo DP26640