On Blocktrades, you can indeed choose to buy a Steem account. You can read more about it in this post.

I’m not 100% sure of the entire reason for having to go there, but one poknt for example is that if you go through the SteemIt account recovery process, you are asked for an email address thzt is attached to your account, while you don’t need an email address when buying an acccount through Blocktrades, so you can’t be sent recovery details