Why do I have to join Live Gold to play the online version of Red Dead Redemption 2...

after what Rockstar did with GTA online I do not have much interest in playing the online version of Red Dead Redemption 2...

Firstly, the retail full version of Red Dead Redemption 2 on disc already includes Red Dead Online so unless you have transitioned to a digital only console, you wouldn't need to purchase it again.
Secondly, which digital version did you purchase? There is the full game Red Dead Redemption 2. There is also Red Dead Online, which is obviously the online part. And then there is Red Dead Redemption 2: Story Mode, which is just a DLC for Red Dead Online (owning Red Dead Online is required) which allows you to play the single player campaign.
Please check your purchase history to see which one you have purchased.

Hi, I hope you're doing great out there despite the issue that you're experiencing. I am an Independent advisor and I am here to help you in regards to your concern.

Just a friendly reminder though that the Red Dead Redemption 2: Story Mode is just an add-on or DLC addition to Red Dead Redemption 2 which means that it requires the base game, you will be needing to buy the base game that is sold separately.

It's not the base game that is needed. It is Red Dead Online. When you state that the base game is needed it sounds like people need to buy Red Dead Redemption 2 but that is not the case. Red Dead Redemption 2 contains both the single player story and the online portion. Red Dead Redemption 2 is not the same as Red Dead Online. They have 2 different names. Story Mode is DLC for Red Dead Online. It is not DLC for Red Dead Redemption 2 as you have said.

Are you sure that you purchased the full Red Dead Redemption 2 and not just the Red Dead Redemption 2: Story Mode. If you purchased the Story Mode, then you need to own Red Dead Online in order to play it. Story Mode is just DLC for Red Dead Online and cannot be played or installed without Red Dead Online.

Here is some information that I found.

Can Red Dead Redemption 2 Be Played Offline?

Rockstar will undoubtedly integrate social components into Red Dead Redemption 2’s single-player story which need an online connection to function. However, players should be able to at least play the story campaign
even if they’re not connected to the internet. An active internet connection will naturally be required for downloading the game’s digital version and for playing Red Dead Online.