It seems like someone did do them! Obvious examples include:
1. basecamp/fogbugz/jira
2. Confluence/Sharepoint
3. django/rails
4. Google Docs
5. Facebook
1. basecamp/fogbugz/jira
2. Confluence/Sharepoint
3. django/rails
4. Google Docs
5. Facebook
Sample walk-through :: Building a Store Locator type Google Maps app over GAE, using iFreeTools : http://blogs.ifreetools.com/2010/11/building-store-locator-t...
Both frameworks support plugins you know.
Not saying you are wrong, just saying I think there is room in that space for some innovation.
Sure, HTML editing is still required and can be painful, but it is an effective way of branding the final result. I think clickety-click interfaces where you modify the interface through various selections / drag-n-dropping are seriously flawed.
Yes, MS Office is a nice example of what a WYSIWYG interface can do, but that's just for standard documents. As soon as you want to get creative in Word, you'll feel the pain and start wishing for HTML.
Besides ... design, other than replacing a logo / playing with colors / choosing a layout from a predefined set, should be left to people that know what they are doing and there is no way around that, because IMHO the hard part is not the HTML language itself, which is included nowadays in high-school curriculum.
It should be built on top of SQL server, or Oracle. People are using it already. It should have a subset of Crystal Reports, with an easy way to develop simple entry yet let you override the business logic if necessary.
Essentially, combine ext-js with django's administration, with a marketplace of addons for various verticals.
As an aside, one thing that these type of tools often lack is integrated testing. Which is a shame, because it seems like it should be easier in these tools, as the tool can have a lot of context about what the fields, etc, mean.