Not hard to see why you'd do it this way either. Just a real quick look at some frame only prices, vs frame plus a build kit, shows that on an XT type spec level, you can save close to $1,000 buying the parts and building it up yourself. Lots of variables in there of course, but if you were to buy as close to like for like as you could, you're going to save a lot of money doing it yourself.
One area where bikes like this are very different to the big names is no proprietary parts, so everything apart from the frame itself is made by a 3rd party. So why wouldn't you build it up yourself, either to tailor everything to what you like, or just save some money.