"What I can't wrap my head around is that making good, efficient software doesn't (by and large) take significantly longer than making bloated, inefficient enterprise spaghetti."

In my opinion the reason we get enterprise spaghetti is largely due to requirement issues and scope creep. It's nearly impossible to create a streamlined system without knowing what it should look like. And once the system gets to a certain size, it's impossible to get business buy-in to rearchitect or refactor to the degree that is necessary. Plus the full requirements are usually poorly documented and long forgotten by that time.