Or it is set out in building codes that they must design in a certain way.

In the UK that means adding lots of insulation. UK houses predominantly had a lot of thermal mass from the inner skin of the cavity wall being brick or later concrete blocks. The little wall insulation, if it even existed, was in the cavity. In a push for more insulation they switched to lightweight thermal blocks, and sometimes more insulation inside, or timber frames. All of which designed for insulation while reducing the thermal mass. No matter how much sun you put in during the day you only heat the air, which goes cold quickly. This is not the architects choice.

Architects can only design for orientation on a single house plot. In the UK they are trying to cram houses on at 50 to the acre or more due to the price of building land. They focus on best use of space, rather than orientation because of that