You can, but then I’d be duplicating my common html in each of the templates and anytime I change one file i’d need to change them all which is unnecessary code duplication. Even with partials, I need a copy of all the same variables used in the partials into each subroute which seems like an anti pattern to me.
Plus, I am using liquid outlets for transitions and I only want the stuff in the outlets to animate, not the whole page. From the perspective of the UI, the outer template remains visually unchanged minus hiding or showing of buttons and or changing of text. When I first started using Ember, I made a wizard using the exact approach you are talking about and I found it to be a lot more work. More typing, more duplication, much more difficult maintenance. New requirements came down and I had to go to each page and change a bunch of common code that really should have only been defined in one place.
The “extra work” only happens once when I decided to implement this solution and took me maybe 15 minutes to do. Now its arguably quicker than per route approach and certainly more DRY. Obviously there’s more than one way to skin a cat but I found this one to be the easiest and most maintainable.