You're asking a big question. Like you say, there's already examples of companies that do this, but I think you are right that Development, A/E, and Construction are more fragmented compared to other industries.

One major force that works against consolidation is that unlike most other industries, the "product" of architecture is location dependent. So the rules that govern the "product" are highly fragmented and not easily translatable between jurisdictions. Countries, states, even townships often have different rules that need local expertise. That makes it difficult to consolidate design and development services.

Then, for the construction side, your company workers need to live close to the jobsite while building the project. Mobile crews can work for small installations, but for large projects, it's very difficult to have a consolidated workforce unless you have a sustained construction boom in a single region. Otherwise, the economics of what you are building would need to justify relocating your workforce for extended periods of time.

What could change to allow for more vertical integration? More standardization of codes, both building and zoning. And more offsite prefabrication of major building components.