Absolutely right. I've gone through the dark times of wondering how to get the linker to stop exploding on me. And when speaking about developing a game engine, it's not just the work required to create feature X, it's the entire ecosystem. Making a sound play or wiring up PhysX is not difficult. Making fifty modules work correctly with each other is the foundation and is not easy or quick. Then there are editor tools, with modern engines providing a full editor suite that would take a team years to make. In the case of Unity, you get correct play-in-editor as well as truly live editing, which allow for incredibly rapid iteration (part of the true secret of efficient game development is being able to go from your shower "ah ha!" moment to a working prototype as quickly as possible). Then there's their asset store with a proliferation of not only art and sound but valuable scripts and entire systems or engines. And while I'm making my game, the middleware company is making my tech better by fixing bugs and adding features, with nearly-push-button support for over a dozen modern hardware platforms to boot.

I can see lots of reasons to develop a game engine from scratch, but they become less compelling every year.