I suppose the owners like single engine/single propeller designs for cost but if I was the captain I think I'd vote for some redundancy.

So would I but apparently the engines have proven so reliable that redundancy isn't required. There are a lot of those container ships out there. I don't recall many of them having problems with engine failures. If container ships were being endangered or lost due to engine failures then they'd be building them with more than one engine.

As Bill Lear used to say, the more stuff you have, the more things there are to break.