... But why?

[Jofs]

Hm

[Ottofyre]

To learn techniques for doing hard surface sculpting in ZBrush.

Or if you meant "Why is this on Steam?" then the answer is because Steam is a digital distribution platform and this is a digital product.

[Wakey]

Because http://store.steampowered.com/news/18952/

[Joviex]

Sorry but I don't agree that Steam is a good place for tutorials, nor should they build it to be.

Making games is not the same as playing/selling games.

If you want professional level approaches, at solid pricing (because if you are doing this you either want a job in the industry, already have one, or want to know what it takes -- so you INVEST in good knowledge first):

Digital Tutors (beginner to intermediate)
Gnomon (advanced -- also for users who are already good artists and are good at picking up new software)
Gumroad (all levels, depends on focus -- DIRECTLY supports the artist)

I am sorry that Steam feels like it should be everything to everyone. That is sad, and counter to the productive effort through which it has blazed a trail -- by focusing and being the best to one area.