I think you underestimate how easy it is to build an external display. When they first released the 5k iMac, there was no interface available that supported an

I think you underestimate how easy it is to build an external display. When they first released the 5k iMac, there was no interface available that supported an external 5k display. Now we have Thunderbolt 3, so in theory 5k displays shouldn't be an issue, but there are only very few options available, and from what I've heard they are plagued by bugs and compatibility issues. So it seems that it is somehow difficult to make external displays that work well. And one thing we haven't talked about yet is price: getting a computer with an integrated display is almost certainly cheaper than getting a computer + a good display. The iMacs have always been pretty good deals if you consider how much a comparable display would cost.