> If you could build the house using only resources that you already have than you would have 100k dollars AND a house.
You still have to spend money on materials and labor - they are never free. You need to take them from other uses, and people may outbid you for them. There isn't infinite lumber and labor lying around the US for free - building houses is expensive!
The core of economics is there is a scarcity of resources and we want to allocate them as efficiently as possible. Money's fungibility allows us to move resources easily and rapidly. I can send you $50,000 much more easily than my car - you might be on a different continent; you might not even want my car or agree on its value, but I can count on you wanting $50K because you can buy whatever you need with it (a car, a few servers, a trip around the world ...).
It turns out in reality that someone with better skills and capital (hard resources, such as heavy equipment) can build a house so much more efficiently than you, that you would save money by hiring them rather than doing it yourself. That's specialization, another economic fundamental. Think of most work - orthopedists, C developers, pipe fitters, etc. They are so much more efficient than you are, you wouldn't dream of trying to do it yourself (on any significant level) - even if you are in the same domain, such as internists or Python developers.
You don't lose out at all, because they also hire you to do what you are specialized at. Then instead of the farmer building a house and the carpenter growing squash, both get better and more houses and squash for much less money.
> If you can take resources from some part of the world that you don't really care about, then you are adding wealth to your own country for basically free.
If you shoot someone on the street, you can take their wallet for 'free'!
... You are arguing against fundemental economics and even basic accounting. If you care about these things, learn about them a little. Microeconomics is actually fascinating, and basic double-entry accounting is essential.