As I see it (I have been in or around construction sites many years, in EU, which have different construction methods/uses - generally speaking - when compared

As I see it (I have been in or around construction sites many years, in EU, which have different construction methods/uses - generally speaking - when compared to the US) the single issue that many people fail to take into account when discussing this topic is that construction (of houses) is not an industry[1], it is artisan work. When (if) you can use pre-fabrication you can speed up things a lot but costs (at the same level of quality) will not be less. Even the single most suited to pre-fabrication sub-unit (the bathroom) won't cost you less than building it on site. Same is true for (specialized) sub-contractors, they are faster (in actually doing their job) but they are usually overbooked, rarely they are on site at the right time and with enough people, so what you gain in time for the (faster) construction you lose in changes to the project scheduling (not much different from your roofing example). [1] there are exceptions for very large developments, but they are rare