Well, u can understand this if I give u one similar example, comparing buying the shoes from the store with making them by yourself (don't forget adding the hours with wage on it)or hiring someone make them for you, which will cost u more money? Obviouly, building a house is more complex than making the shoes, again, it depends on many many factors, for example, if ur salary is $ 50,000 a year, and building a house by urself(usually means hiring a construction company to build for u), the time and energy u spend on it, i guess at least half year, so u have to add $25,000 on the costs of your house.
however, if u want to build a very expensive house (over 1 million, for example), that is another story, maybe u will save some money if u build by urself.
I am not an expert on it, but I am very interested in this kind of topic.