Actually the reason was because of the varied dog sized, because of it, they will need to create a huge object just for the dog house. IT might be even bigger compared to the previous iterations since The Sims 4 is scaled a bit bigger compared to the previous iterations. For example in the livestream they talked about it, such as how they will need to do doghouses since some dog breeds such as a chow chow is so big. Would rather use their resources to another gameplay feature rather than to an another oversized object.