@vitor pretty much sums up the reasoning behind this. Essentially, Alfred is extremely lightweight on a user's Mac, having minimal CPU and battery impact at all times. This is essentially achieved by only running the internal logic paths required which lead to the user's desired result at any one time, not running any unnecessary code. This becomes more significant when [non-native] workflows may take a decent amount of CPU or network traffic.
Added to the fact that this is a rarely requested feature, it's unlikely that this will be implemented into Alfred's default results.
Having said that, and having a think about this, there is one aspect I would consider. You can already create a workflow and attach it to an input filter (such as a file filter, script filter etc). At this point, it's a 1 to 1 mapping for hotkey to input filter. If there was interest, I'd look at enhancing this so that one hotkey could connect to multiple input filters, and results from all filters would be shown. This would essentially allow you to build a workflow which shows a custom version of Alfred with the specific, highly customised results you'd like.
