What do you think it means to say something has a steep learning curve? Does “tic-tac-toe is a game with extremely steep learning curve” sound right? If it doesn’t, you probably have been using the phrase the wrong way all your life, or you don’t know what tic tac toe is.
Learning curve plots learning (skill) against experience (time), so if something has a steep learning curve, it means that you can do a lot of learning with relatively little experience (time).
The problem is the term conjures in the mind the idea of scaling a steep cliff as a metaphor for learning a difficult task. Another reason is probably due to the ambiguity of the phrase. If the graph was plotting experience against learning. Then a steep learning curve would mean spending a long time with little progress. However, the long term asymptotic graph would be a vertical line which can be quite confusing.
Yet another reason (after some discussion with a friend) is it can be confused with “something requires a steep learning curve”, which means that a lot has to be learned in a very short time.
In conclusion, the popular meaning of “steep learning curve” is “difficult to learn”; the technical meaning is “quick to learn”.