It's all narratives. Clearly Caleb is up there already, even as he is making lots of mistakes still. All the recent QBs we've had were nothing like Caleb is right now, imperfect as he is.
The reason the word "generational" is thrown around is that we can clearly see that Caleb has LOTS of room for improvement, and yet he's still on pace to shatter the all time best season by a Bears QB. after 2 years, Caleb as a rookie put up the 5th best season, and he is on pace to have the best season as a passer in Bears history.
Right now, the Bears offense is 3rd in the league for yards. Our Passing yards is 10th, and our Running yards is 2nd. We are 6th in scoring.
And remember those first two weeks where we were awful? They account for 1/4 of those stats too. That's how good Williams and Johnson have been.
Some guys are just mediocre. Caleb is amazing and great, but he is still making a lot of mistakes, so he averages out to mediocre. The consistently mediocre player wont get better, but when Caleb becomes more consistent, and makes fewer mistakes, he is going to be elite.