I'm pretty sure the electron browser engine moves slower than Blink.

If so, it would be extremely unhelpful for development and debugging since you would rarely have the latest engine on your machine, which would be the engine the majority of your customers would be on.

Using an external browser makes a lot of sense in that regards, and also, it separates the testing engine from the application itself, which allows for (although I doubt it's been implemented yet) testing in different browsers, or different versions of browsers, etc.