The first reason is that LLM advancements are yet to stop. And the second reason is that "the current tech" is a moving target.
If, let's say, in 2029, it turns out that autoregressive transformer LLMs have exhausted their potential, the R&D that goes into improving them now would be put into finding alternatives. And I struggle to imagine not finding any.
Why do you assume there is no hard limit we’ll hit with the current tech that prevents us from reaching AGI?