The world keeps moving on, and so the models need to be retrained so that they can keep up with new information. Otherwise you'll get stuck with a model that only works well with information that existed prior to a dataset horizon that's receding into the past at a constant rate.
At the same time, they have to keep iterating on the training process itself. AI generated text and code is slowly spreading across the internet. Model collapse is a real concern; they wouldn't be spending quite so much energy on buying and scanning rare books if it weren't. But for coding in particular expanding their corpus of old text is not really a good option because of the previous problem - no good training your LLM to write 1980 vintage K&R C that won't even compile on a modern compiler.