There are legacy parts of browser layout that make it impossible(?) to optimize some rendering while being standards compatible. If you just chose a subset that is fast to implement and focused on being subset you narrow the scope of the project and enable optimizations.
Electron has to implement everything chrome has to support even though the apps running in it will never touch it.
Also web engines have to be heavily sandboxed because you cannot allow internet code break out of the sandbox.
Native apps shipped with electron already ship Node and just waste resources sandboxing stuff with security layers.