I also like building bikes from the frame-up.

It helps that based on experience I can usually figure out exactly what I want before starting, and that I have the mech. skills and tools to work on pretty much anything that doesnt require machining/brazing.
I also like to look for cool used and vintage components, refurbish and use them where possible which can save a bit of money... A $25 1980's/90's MTB can supply a fair share of a touring build, even on a much nicer frame.

I cannot imagine what it would cost to double-buy a lot of expensive components, or pay someone to do the wrenching.

The added benefit of building your own is better maintenance and reliability of your bike. Problems are more likely to be noticed and identified.