It's a great question, and I have given this issue a lot of thought.
The reason is that Q2A is changing very quickly, so a template-based theming engine (where the theme consists of HTML code plus placeholderes) would very quickly fall behind enhancements to the platform. By using an object-oriented theme architecture (where the theme only overrides small parts of the HTML), a theme is much less likely to break when a new version of Q2A comes out.
When Q2A hopefully settles down (perhaps around version 2.0?) it will be the right time to consider a template-based theming engine.
BTW, the other reason is that there are tons of options in Q2A to show/hide different pieces of information (e.g. post times, user points, etc...). This is hard to do with template-type themes, unless of course you use a templating language like Smarty with if/else structures and the like. But in that case it probably just makes sense to stick to basic PHP, so people don't have to learn two languages.
To summarize, I haven't found a way to make theming as easy as it is in WordPress, while keeping the flexibility that Q2A currently needs. I'm very much open to suggestions though...
