LG did negotiate with YoutubeTV, which is in their app list, which we pay for. Through the YoutubeTV app you can buy HBOMax. Someone in LG either did not check the YoutubeTV content details or missed the fact you don't receive the full product. If you use your log in on hbomax.com on your computer or their phone app, you can receive the entire product, just not on the LG OLED tv.
Either LG didn't pay attention, or received a better price for the product that seems to include HBO now, or they didn't want to spend the time integrating HBO MAX into their programing infrastructure because, in the future, they may deal with HBO Max directly.
My 2 cents