The software (actually, any software) is created around some basic principles. Those principles reflect the authors’ perceptions of proper ways of work, among other variables.
There are two main approaches to creation and working with electronic documents. One of them is providing a means to work with electronic document as with a paper and a pen. This approach often more “intuitive” for new users, and allows for less initial learning to start creating documents that look as user wants.
Another one is providing means to work with electronic document as some structured data, that reflects the document’s information structure. This approach emphasizes the information value of the document, and allows for better integration of the data/information into greater informational space of today’s world. But this requires more initial learning.
These approaches are mutually exclusive actually. They require different tools, and different state of mind. They require different architectural decisions.
Your question is about providing means for users of first approach to work more easily with the software. And each such proposed means must be weighed against its pros and contras.
What pros do the suggestion have? Well, obviously: making some operations easier for some people.
But wait, what contras could such a great thing possibly have? Actually, many. If you think about the basic approaches outlined above, any feature may help users of one or both approaches, or not affect one or both, or impede one or both. Your suggestion is beneficial to first approach users; but what about those who use second? “Well, if they don’t want to use manual formatting, then let them just not use it!”? No. The option, if implemented, would promote the manual formatting way, and decrease incentive to learn the way that is considered the proper one by those who create the software. This will increase document in the wild that have improper structure, and make life of those who do it the right way more difficult, because they will need to convert improper documents (created by ones who don’t think about further destiny of their “creations”) to proper structure.
This software (LibreOffice) is built around the second approach. And you may deem everything you want as “unnecessary complication”. It’s your choice. But for those who create the software around these principles, your proposal must be unnecessary complication of the target user base. Not only would they need to create and support some new code that takes time and manpower, but also impair life of those who consider principles of LibreOffice as its high strength.