I haven't built a home but I've built quite a few large buildings from kits. Generally you get the raw lumber shipped to you in a big pile that you must sort through to find the correct pieces, cut to size and use where specified in the directions. The advantages is that someone has done all the raw material calculations for you, they give you a plan for how to build the thing and it gets delivered on site so you don't have to worry about hauling everything. However they do come with some disadvantages. There is often lots of ambiguity in the directions so you cut the wrong piece to size and end up with something too short to work later. Or sometimes you simply just use more than what was called for, etc. Another thing is that sometimes several of the boards shipped to you might be split, badly warped, etc. to the point of not being usable and so you still have to go buy some lumber to finish up. Finally, with kits, you generally get everything all at once which means you have to protect it from the elements or opportunistic thieves until used. If you are taking many months to say build your own house, that is a long time to protect it and it is much easier to have it delivered closer to when you actually need it.
Kits definitely have their place in the world but they certainly aren't for everyone or every site.