Your view code shouldn't go into your view controllers, it should go into your views. This is a mistake that is very common among iOS engineers. Anything dealing with subviews, layouts, touches, etc should go into subclasses of UIView; view controllers should handle touches (delegation!), interact with the navigation controller, and act as a bridge between your views and the rest of your application code.
But why make UIView subclasses when you don't have to? If I'm just using a tableview, and I need to lay that out, I'm not going to subclass UITableView. I'm just going to add one to my controller or use UITableViewController. If you're already making subclasses of UIView, then yes, do the layout code there, but for many applications this isn't necessary, and there's no where else to put that code.
Well, maybe. UITableViewDelegate and UITableViewDataSource don't have anything to do with the layout of that tableview. But let's say I have a bunch of labels, or textfields, or buttons or imageViews. All of those stock classes will do a pretty good job without subclassing, and for many applications, there's no need to subclass them. If you're doing your layout in code, where else does the layout code go other than the Viewcontroller that manages all those objects.
I'm an iOS developer and I generally do what you suggest here. I create a bunch of subclasses of UIView and do my positioning via layoutSubviews, implement sizeThatFits when appropriate, etc.
Despite this I feel like Apple makes it very difficult to keep all UI code out of your view controllers. I'd go as far to say they actively encourage shoving UI code into your UIViewControllers. Take a look at the interface for UIViewController -- it is almost all view/presentation related [0]. Most of Apple's sample code and projects have view code sprinkled all over their UIViewControllers.
I feel like it is a constant fight to keep proper view/controller separation on the iOS platform.
Except UIViewController has a view property that is usually a container for a lot of other views. Sometimes it's desirable to add constraints to this view. Apple even provides API for this: UIViewController's updateViewConstraints method. I think of this as a compromise so we don't have to create a custom UIView subclass for every UIViewController subclass.
