Nope, it is simply because they are overwhelmed. Either it's too much work to do after your day job or just too much work for one person.

Dunno, this "harder" argument while valid sounds just like false security. That's why I don't see much weight in it.

As for bandwidth, source maps are not automatically pulled from server, so it also seems like a false issue to me.

That's the thing, it's not actually a security measure. Security through obscurity is not security. It can provide false security impression that is more harmful in my opinion.

Having source maps can encourage proper security practices. Which, in my books, very much outweighs any security benefits of hiding them.