Believe it or not, I ACTUALLY KNOW THE FUCKING ANSWER NOW. It happened to come up over Christmas with a relative who works in construction.
I thought it was just stupidity... laziness. Clear cut it all and build. But there actually is a slightly more tolerable reason:
Nowhere here is flat.
So take this bunch, for example:
They flatten the terrain to create a good surface for building homes. Doing this means they have to remove a lot of soil from the uphill side, and they have to buy a lot of fill for downhill side.
Removing all of the trees and making all of the land flat is simply the cheapest, easiest way to do this because it creates a shitload of free fill that they can use. So the right side of the properties above, they had to remove a tonne of earth to get that ground flat. And they added it to the left side to raise that up so the whole property was flat.
If they left the trees, it couldn't be flattened. And by doing it this way - free fill. Saves a fortune. Seriously... every time a new project is started here, they're a year just blasting into the bedrock trying to get the ground flat. We've only got a couple of inches of "soil" in most areas. And at the elevation of the example houses above, there would've been basically no soil before they were there. Exposed rock with moss and shrubs and trees in every nook and cranny.
As for why they don't then plant saplings - some do, but the landscaping packages are expensive and optional so many homeowners opt out. However, lots of private homeowners do add their own. That said, there's also a cultural thing that trees = living in the woods = being a hick. You'll hear people look at homes with gorgeous trees out front and say things like, "Look at that, sure the branches are almost touching the house. That's poverty."
Everyone here loves trees in their backyard, though. That's where they're usually found.