Take this shot of Morton Stanley by Mike Campbell - one of my favourites of the layout... except 20% of the shot is other stuff. Enclosing the scene would be better.
Part of the reason is the finality of boxing-in. In my head there is always the possibility to extend and although it is possible to rip end boards off, it usually happens at 8.30 am on the morning of a show. This doesn't end well and neither does the properly intended action - there is always debris and it never is as clean an operation as you first wished. I realise this is fairly old hat thinking in some quarters, and I first built a fully boxed layout in 1989, so hardly new for me either, but there is a step change in thinking that needs to be taken.