For starters, a lot of features add negative value. For most things, people don’t want everything and the kitchen sync thrown it. It confuses them and makes the product hard to use. If your model were true, any dummy off the street could achieve linear growth in a product by just continually adding more features. That’s not how it is.

Second is what you hit on. Some features add exponential or greater value.

This means that the value of new features does not fit a bell curve and reasoning about it with averages is a mistake.