Carrot or carrot negotiating tactics - novel.It seems to me that is exactly what Rubio and Kellogg are doing and why they are meeting without Zelensky. The US and Russia look to establish a relationship for the first time in 80 years. Something Trump wanted to do eight years ago but was forced not to pursue due to Russia Gate.
Some of the points could be the US drops sanctions against Russia and normalizes the relationship allowing Russia back into the G8. The construction of the new NATO base in Romania stops and Ukraine will not join NATO. Russia has to offer a reasonable treaty to Ukraine and start pulling back from it's new relationship with China, further isolating China.
Zelensky and Ukraine are free to accept/negotiate the treaty or continue the war without US support and Europe is free to do what they want.
I don't know if this is what is happening but it seems the logical reason why Zelensky is not involved in the first round of talks. The second round of talks with America, Russia and Ukraine deals with a treaty.