My guess security of GitHub Pages is the reason for using Liquid in Jekyll. For generating static pages on your desktop before uploading Liquid is simply not needed.
I am a bit surprised jekyll is so heavily promoted as a static site tool(saw it twice on HN front page already) while an elegant and seemingly more powerful ruby application (webby) already exists for this purpose.
There are differences in the philosophies behind the 2 projects. They are both static site generators however Jekyll is more oriented for maintaining blog-style sites. Webby lacks the ability to organise content the way that Jekyll does.
Perhaps most importantly, Jekyll is now available as a free, hosted solution with GitHub - a great way of inviting the large community of developers active in that community to contribute to the project.
I am not saying one is better than the other. They seem to accomplish slightly different things although the method in which they go about it is somewhat similar.
You can create a blog with two commands in webby:
$ webby-gen blog "My Blog"
...
$ webby blog:post "Nice weather today"
[13:27:47] INFO: creating content/blog/2008/index.txt
[13:27:47] INFO: creating content/blog/2008/12/index.txt
[13:27:47] INFO: creating content/blog/2008/12/28/nice-weather-today.txt
And it obviously does support categories/tags/article statuses etc. since you can give any property to your page.And since everybody thinks the GitHub guys are cool, lots of people are going extend Jekyll with all kinds of features that Webby already got.