There's a good reason for this, which I will explain with an analogy:
Lesson 1: Write the Great American Novel.
Lesson 2: Learn how to write engaging, readable short stories.
Lesson 3: Learn how to write internally consistent essays.
Lesson 4: Learn how to write coherent paragraphs.
Lesson 5: Learn how to structure grammatically correct sentences.
Lesson 6 Learn how to use and spell common English words.
Lesson 7: Learn how to enter text on a keyboard.
Get it? Lessons have a natural order, and each lesson requires what was learned beforehand.
Also, I wanted to browse around the lessons in order to evaluate the quality of the site so that I could decide whether to recommend the site to non-programmer friends.
That should have been the first thing you said. :)
But such tutorials are obviously for rank beginners, experienced programmers don't use these kinds of sites. Unless it's a dramatic jump, like from COBOL to Haskell, to learn a new language they go straight to the language's documentation:
Python 2:
Python 3:
With Codecademy I could do that, here I cannot. So Codecademy it is.
Skipping is a really important feature for those who you hope would promote your service for you :-)