Why hasn't prefab construction taken over?

It's nice and easy to modify and repair also. Unfortunately most of the money people use to buy goes into the price of the land, leaving little extra for a quality build.

Certain type of prefab help, i.e. things that would normally stall a project, or things that are generally the same. For example, a new house I just bought uses some prefab concrete plates where the amount of variation is limited. On-site, some minor changes were made and it sped up a lot of the foundation, walls and floors.

Other elements, like entire frames with windows and doors etc. are 'prefab' as well when you think about it. We have had those for years, but it didn't always be modular like that.

The real 'custom' stuff so far is the roof, which is a well-known pattern and construction method, but the final size and shape depends on the house. Same goes for details in your walls like where the doors and windows go.

If you optimise 'too much' for some of the construction elements you stand to lose more than you gain (i.e. now you have to keep a larger stock of prefab stuff due to the amount of variations -- or people might end up with a choice biased towards something that is easy rather than what they actually want). It's also easy to think that a complete module could be prefab and the design re-used over and over, but that only makes sense if all houses are the same and all land is the same (Which is not the case). Then you end up with prefab stuff that doesn't fit and you have to either live in a crappy not-really-fitting-well house or do a lot of (more expensive) local changes at installation time.

Having some re-usable modules is the way to go if you want to optimise; especially when you think about things that are similar in a lot of cases (be it by choice or by building laws). We're already doing a lot of that, and optimising it further (be it selection or scale) would probably gain you more:

You can design with ready to use components where applicable, you can build with known-good components, methods and time scales where applicable and as the one buying the house you still get to make whatever choices you want without the risk of simply getting a 'badly modified prefab'.

11% of global CO2 emissions are from construction. Using pre-fab steel modules are not only more recyclable but the building process emits about 50% less CO2 as well. Building in earthquake resistance is much easier than steel-reinforced concrete. So it's not just an optimization on on-site time.

Unfortunately no translated captions in https://www.bilibili.com/video/BV1mK411p7X4 but the wall-to-wall robot in the factory gives you an idea of what the future looks like.