Building up reuses existing structure; foundations, walls, … . Building out has nothing to reuse. There are also paperwork costs, as the land around your house might have stuff in it that needs to be relocated and that means more cost for drawings and digging and stuff. Not to mention that you might need a variance from some zoning rule (which sometimes is needed to build up, so that’s not always a deciding factor).
If you’re in a city center, land is scarce and insanely expensive, especially the particular land that happens to be next to the land you already own. So you have to make the most of the land available by building a very tall building on it.
If you’re out in the country, it might be more economical to build a sprawling bungalow
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