Why pay for labour when people are willing to do it for free, *and* procure a measure of prestige for themselves for being part of the construction project? It's not like the old days where any old JW could just walk onto a building site either for a Quick Build™ Kingdom Hall™ or Assembly Hall™ - now every person on the site must be vetted as to their skill set and their Spiritual Qualifications™.
If the WTS can get their labour at no cost, and materials at cost or donated, they will take it. It's part of their Take-Take-Take modus operandi.
As an aside, they seem to be far more concerned about the qualifications for anyone participating in a building project than they are ensuring that people coming in contact with children are safe to do so.