I see, so you need way more knowledge to get a small increase in reward, hence the steepness. Point taken.

Edit: Wikipedia though

A learning curve is a graphical representation of the relationship between how proficient people are at a task and the amount of experience they have. Proficiency (measured on the vertical axis) usually increases with increased experience (the horizontal axis), that is to say, the more someone, groups, companies or industries perform a task, the better their performance at the task.[1]

The common expression “a steep learning curve” is a misnomer suggesting that an activity is difficult to learn and that expending much effort does not increase proficiency by much, although a learning curve with a steep start actually represents rapid progress.[2][3]