The most likely reason for this is how AI model training work. Depending on the model’s complexity, training data size etc, it can enornous amounts of time to finish a training. Probably the initial training must be atleast 2-4 week at Google if but that’s just a huge assumption.

After that they probably train this base model with some newly acquired data (ex: 1 week of data) which won’t take as much time to finish compared to starting from 0 all over again.