Actually, I think the reason is that the developers are greedy, and it costs them less to bulldoze and build (cheaply) anew than to restore what already exists. You see it in microcosm when people buy houses around here: the good ones take the time to fix what's there; the jerks tear 'em down and put up something new instead.
I also think many developers have no stake--they think--in preserving the existing community because they don't think there's enough money in it for them. Often, of course, they find that by disrupting a community's ecosystem (for want of a better word), after a short spell of extreme profit, they see no returns at all, because, as you say, Gilly, the new Frankenstein community is not sustainable.
My local developer is fifty years old; he grew up in New Brunswick (Canada) and Aroostook County, though he arrived here after years of being a creep in Florida. He knows, or at least once knew, what a community looks like and, specifically, what a community in rural, coastal Maine looks like. He claims to value the community here. But he also claims he knows what's best for it, and what's best, apparently, is what he hopes will make him the most money. Thus, as I type, the factory that he claimed he had every intention of maintaining as a working waterfront venture is being gutted and fitted with apartments (though meanwhile a lobster bait concern remains in one portion--have you ever smelled lobster bait? just what someone purchasing a high-priced condo wants next door). My one satisfaction is that he hired one of the worst carpenters in town, a young fellow whose work ethic also suffers somewhat from his addiction to oxycodone, so the resulting units will likely prove to be jerry-built pieces of shit for which Mr. Victor "Lying, Greedy Bastard" Trafford will not be able to get the no doubt exorbitant prices he intends to charge.
