You could read it and understand it because it was the same language you used. If there was a problem you could debug it.
I never felt more productive than when my UI declaration was just code.
Inevitably you need to mix some logic in the layout language and then debug it (For each render this thing, binding syntax etc.) now you have a weird programming language that is hard to debug and remember syntax for, or you could just use a normal programming language, maybe one that supports some declarative constructs.
SwiftUI gives me hope, would like to see something similar show up for the .Net World.