Answer: Extrapolation
Extrapolation only works if the data is truly fitting of some function or pattern. Often, we won't know what the function for a scatterplot will be so making extrapolations could lead to errors.
Example of extrapolation working: You drive for 1 hour and travel 60 miles, 2 hours gets you 120 miles, 3 hrs you go 180 miles. You can successfully extrapolate that you'll go 240 miles if you drive for 4 hours (assuming you keep the speed up). The formula that fits the data is y = 60*x with x as the time in hours and y as the distance traveled.
Example of extrapolation falling apart: It rained yesterday and it rains today; therefore it will rain tomorrow. The flaw here is that it might clear up tomorrow and not rain at all. Unfortunately weather isn't that simple. There are a lot of factors at play.