I was the one suggesting Habitat for Humanity and I agree with you that it isn't the most efficient way to support home construction. This was one of the reasons that I had to be persuaded to go.
Obviously, professional construction workers would build homes faster and better than us amateurs. On the surface, it appears that I could simply have done what I am good at (software) for two weeks and pay professionals to do the building in Hungry instead.
While working in Hungry I had time to reflect upon this for two weeks. Most of the volunteers there did it because they wanted to help people, and most didn't think about the economic efficiency of they way they were doing it. A few of us (some Stanford MBA's and I) talked about it.
In retrospect, the issue is more nuanced. Habitat for Humanity is the world's largest non-profit homebuilder [0]. They require the future owners to contribute a significant portion of the effort in construction of what will become their home. This, they claim, results in better care for the home after they move in. If so, it seems like a useful model (but see the criticisms of Habitat for Humanity on the cited Wikipedia page).
Couldn't this be done using professional builders and not volunteers? Perhaps. However, let me relate something that was said to me in the small town where I was working. We were building a home for a family of Romani Hungarians in eastern Hungary. These people, sometimes called Gypsies, are not always treated well in rural areas of Hungary[1].
At one point, I met a townsperson that spent some time speaking to me. He said that at first the townspeople couldn't understand why people from the USA had to go to Hungry to find work. They were surprised that we were not being paid but instead had to pay for the privilege of working in Hungry (there are local trade's people involved with the construction that must be paid, construction supervisors, electricians, plumbers, cement companies and materials have to be paid for by our donations).
He told me that the concept of charitably taking care of someone not a family member was unusual there, and the whole town noticed us being there and doing it. We were well treated and the people were very friendly to us. One benefit of the trip is that we gained an appreciation of a different culture and so did they.
The remarkable thing was that he (the townsperson) described to me how seeing us volunteering to help people we didn't know in his own town inspired him to do the same; he had actually taken time in the past to work on the build too after seeing the volunteers from the US doing it. I realized that it isn't just about economic efficiency.
I still have mixed feelings about Habitat for Humanity, but overall I continue to believe that it is a form of "volunteer tourism" that has pluses and minuses but ends up being a net positive.
[0] https://en.wikipedia.org/wiki/Habitat_for_Humanity
[1] https://en.wikipedia.org/wiki/Romani_people_in_Hungary